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LeetCode Questions And Answers

#	question	Answer	level
001	two sum	Java - without hashMap Java space O(n) time O(n) - with HashMap Java - space O(1) time O(n*2)	easy
002	add two numbers	Java - O(n) Java - O(n) better code	medium
003	Longest Substring Without Repeating Characters	Java - sliding window Java - another	medium
004	median of two sorted arrays	Java O(log(m+n))	hard
005	longest palindrome substring	Java O(n^3) Java O(n^2) space O(n^2) Java O(n^2) space O(1)	medium
006	zigzag conversion	Java	easy
007	reverse int	Java Java - Better	easy
008	string to integer	Java Java - AutoMata	easy
009	palindrome number	Java	easy
010	regular expression	Java O(N^2) DP	hard
011	container with the most water	Java space O(1) time O(n)	medium
012	Intger to roman	Java	medium
013	Roman to Integer	C++ Java	easy
014	longest common prefix	Java Java better solution Java - divide and conqure	easy
015	3 sum	Java	medium
016	3sum closest	Java	medium
017	letter combination of phone number	Java - recursive Java - DP Java - backtracking	medium
018	4 sum	Java	medium
019	remove Nth node from the end of the list	Java - O(n)	easy
020	valid parentheses	Java - solution 1 - stack Java - Array as stack -- much faster	easy
021	merge two sorted linked list	Java - iter 2 Java - iter 3 Java - Iter Java - recurrsive Java - right but no good solution C++	easy
022	generate parentheses	Java - DFS	hard
023	merge k sorted listnodes	Java - merge sort	hard
024	swap nodes in pairs	Java - Iter Java - Recurr	medium
025	reverse nodes in N group	Java - O(n) Java - recursive	hard
026	remove duplicates from sorted array	Java - solution 1 Java - solution 2	easy
027	remove element	Java Java - right, but not perfect coding	easy
028	implement strStr	Java - KMP O(n) Java - brute force	easy
029	divide integer	Java - iterative Java - recursive	medium
030	Substring with Concatenation of All Word	Java	hard
031	Next Permutation	Java	medium
032	Longest valid parenthesis	Java-stack	hard
033	Search in Rotated Sorted Array	Java	hard
034	search for a range	Java	medium
035	Search Insert Position	C++ Java - O(n) Java - O(logn)	easy
036	valid sudoku	Java Java - O(n*n)	easy
037	sudoku solver	Java - backtracking	hard
038	count and say	Java	easy
039	combination sum	Java - backtracking Optimized Java - Not Optimized	medium
040	combination sum II	Java	medium
041	find missing positive	Java - O(n) Java - O(n)	hard
042	trapping the rain water	Java - time O(n^2) span O(1) Java - DP time O(3n) span O(n) Java - two pointer time O(n) space O(1) Java - stack time O(n) space O(n)	hard
043	multiply strings	Java Java - better	medium
044	Wildcard Matching	Java	hard
045	Jump Game II	Java - Greedy O(n) Java - DP O(n^2)	hard
046	Permuation	Java - backtracking	medium
047	Permutation II	Java	medium
048	rotate image	Java - In place	medium
049	group anagrams	Java Java - sort str	medium
050	power(x,n)	Java Java	medium
051	N-Queens	Java - Backtracking	hard
052	N-queens II	Java	hard
053	max subarray	Java - space O(1) Java - space O(n) C++ D&C	medium
054	Spiral Matrix	Java	medium
055	jump game	Java	medium
056	merge interval	Java	hard
057	insert interval	Java	hard
058	length of last word	Java Java - From back	easy
059	Spiral Matrix II	Java	medium
060	next permutation	Java	medium
061	rotate list	Java	medium
062	unique path	Java - space O(m*n) Java - space O(n) Java - space O(1) math C++	medium
063	unique path II	Java	medium
064	minimum path sum	Java - In place Java - O(mn) Java - O(n) C++	medium
065	valid number	Java	hard
066	plus one	java Java - another	easy
067	add binary	Java	easy
068	text justification	Java	hard
069	sqrt(x)	Java - newton Java - binary search	easy
070	climb stairs	Java - recursive Java - DP O(n) Java - DP O(1) C++	easy
071	simplify path	java	medium
072	edit distance	Java - O(m*n)	hard
073	set matrix zeros	Java	medium
074	search a 2D ma"trix	Java Java - binary Search	medium
075	sort colors	Java C - two pointers C - count&replace	medium
076	minimum window substring	Java - two pointers, O(n)	hard
077	combinations	Java - backtracking	medium
078	subset	Java	medium
079	word search	Java	medium
080	remove duplicate from sorted array II	Java	medium
081	search in rotated array II	Java	medium
082	remove duplicates from sorted list II	Java - O(n)	medium
083	remove duplicates from sorted list	Java - Ugly Java - Pretty C++	easy
084	largest rectangle in histogram	Java - O(n) Java - Stack Java - divide and conqure	hard
085	maximal rectangle	Java	hard
086	partition list	java	medium
087	scramble string	Java -recursive	hard
088	merge sorted array	Java	easy
089	gray code	Java C	medium
090	subsets II	Java	medium
091	decode ways	Java - DP O(n) Java - DP O(1)	medium
092	reverse linked list II	Java	medium
093	Restore IP Address	Java - backtracking	medium
094	binary tree inorder traversal	Java - recur Java - iter Java - Morris C++ iter C++ recursion	medium
095	unique binary search tree II	Java	medium
096	Unique Binary Search Trees	C++ Java - DP	Medium
097	interleaving string	Java - DP	hard
098	validate binary search tree	Java - recursive inorder Java - recursive Java - iterative inorder Java - recursive with boundary	medium
099	recover binary search tree	Java	hard
100	Same Tree	C++ Java - recursive Java - iterative	easy
101	symmetric tree	Java - iter Java -recur C++	easy
102	binary tree level order traversal	Java - stack Java - queue Java - recursive	easy
103	binary tree zigzag level order traversal	Java	medium
104	Maximum Depth of Binary Tree	C++ Java Java - iterative Java - DFS iter	easy
105	construct binary tree from inorder and preorder traversal	Java	medium
106	construct binary tree from inorder and postorder traversal	Java	medium
107	binary tree level order traversal II	Java C++	easy
108	convert sorted array to binary search tree	Java	medium
109	convert sorted list to binary search tree	Java - cheating, use array Java	medium
110	balanced tree	Java - O(n^2) Java -O(n) C++	easy
111	minimum depth tree	Java - BFS Java - DFS	easy
112	path sum	Java - recur Java - queue Java - stack	easy
113	path sum II	Java - recursive Java - backtracking	medium
114	flatten binary tree to linked list	Java - recursive Java - iterative	medium
115	distinct subsequences	Java - DP, O(m*n) Java - DP O(N) space	hard
116	Populate Next Right Ptrs In Each Node	C++ Java Java - O(1) space	medium
117	populate next right pointer in each node II	Java - space O(N) Java - space O(1)	hard
118	pascal triangle	Java - recurrsive Java - Iterative Java - better recursive	easy
119	pascal triangle II	Java space O(n) Java - new solution	easy
120	triangle	Java Java - Bottom Up	medium
121	best time to buy and sell stock	Java Java - improve Java - space O(1) time O(n)	medium
122	best time buy/sell stock II	Java - time O(n), space O(n) C++	medium
123	best time to buy and sell stock III	Java - Space O(n) Java Space O(1)	medium
124	binary tree max path sum	Java	hard
125	valid palindrome	Java Java - regex	easy
126	word ladder II	Java	hard
127	word ladder	Java Java - graph	medium
128	Longest Consecutive Sequence	Java Java	hard
129	sum root to leaf numbers	Java - 1 Java - 2	medium
130	surrounded regions	Java Java - union find	medium
131	Palindrome partitioning	Java - backtracking	medium
132	Palindrome Partition II	Java	hard
133	clone graph	Java - BFS Java - DFS	medium
134	Gas Station	Java	medium
135	candy	Java - space O(3N), time O(3N) Java - space O(1), time O(N)	hard
136	single number	Java - time O(n) space O(1) Java - HashSet C++	medium
137	Single Number II	C++ Java Java O(n)	medium
138	copy list with random pointer	Java - wrong Java - O(n) time O(1) space	hard
139	word break	Java	medium
140	word break II	Java -DP	hard
141	Linked List Cycle	C++ Java	medium
142	linked list cycle II	Java	medium
143	reorder list	Java	medium
144	binary tree preorder traversal	Java - recurrsive bad Java -recursive good Java -iter Java - Morris C++ iter C++ recursive	medium
145	binary tree post order traversal	Java - recurr	hard
146	LRU cache	Java	hard
147	insertion sort list	Java - O(n^2)	medium
148	sort list	Java	medium
149	Max Points On a Line	Java	hard
150	evaluate reverse polish notation	Java Java - Better OOP	medium
151	reverse words in string	Java	medium
152	maximum prod subarray	Java - DP Java - two arrays	medium
153	find min from rotated sorted array	Java Java - better	medium
154	find min from rotated sorted array	Java	hard
155	min stack	Java Java - Single Stack	easy
156	binary tree upside down	Java - recursive Java - Iterative	medium
157	read N chars given read4	Java - solution	easy
158	read N chars given read4 II	Java - solution 1 Java - solution 2	hard
159	longest substring with at most two distinct char	Java - HashMap Java - Array	hard
160	intersection of two linkedlists	Java - 1 Java - 2	easy
161	one edit distance	Java - solution1 Java - Solution2	medium
162	find peak ele	Java -O(n) Java - O(log(n))	medium
163	missing ranges	Java - O(n)	medium
164	Maximum gap	Java - nlog(n) Java -O(n) bucket sort	hard
165	compare version	Java	easy
166	Fraction to recurring decimal	Java	medium
167	two sum II	Java - two pointers	easy
168	excel sheet column title	Java	easy
169	majority element	Java - HashMap Java - sort Java - space O(1) Java - Bit manipulation C++	easy
170	two sum III	Java - add O(n) find O(1) Java - add O(1) find O(n)	easy
171	Excel Sheet Column Number	C++ Java	easy
172	factorial trailing zeros	Java - log(n)	easy
173	binary search tree iterator	Java	medium
174	dungeon game	Java - DP, memorization	hard
175	combine two tables	MySQL - left join	easy
176	second highest salary	MySQL - Offset	easy
177	Nth highest salary	MySQL	medium
178	salary rank	MySQL
179	largest number	Java	medium
180	consecutive numbers	MySQL	medium
181	Employees Earning More Than Their Managers	MySQL	easy
182	duplicate email	MySQL	easy
183	customoer who never order	MySQL	easy
184	highest department salary	MySQL - Join & In	medium
185	department top three salaries	MySQL	hard
186	reverse words in string II	Java - Solution	medium
187	repeated DNA sequence	Java Java - Rolling Hash	medium
188	best time to buy and sell stock IV	Java	hard
189	rotate array	Java - reverse O(3n) Java - O(n)	easy
190	reverse bits	Java - 1 Java - 2 Java - better solution	easy
191	Number of 1 Bits	C++ Java - 1 Java - 2	medium
192	word frequency	bash - tr, sort, uniq, awk	easy
193	valid phone number	bash - grep	easy
194	transpose file	Bash	medium
195	tenth line	Bash	easy
196	delete dup emails	MySQL	easy
197	rise temprature	MySQL	easy
198	house robber	Java DP Java better code C++	easy
199	binary tree right view	Java - BFS Java - recursive	medium
200	number of islands	Java - DFS	medium
201	bitwise AND of range of nums	Java - brute force Java - bitwise AND	medium
202	happy number	Java C++	easy
203	remove element from linked list	Java
204	count primes	Java	easy
205	isomorphic string	Java Java - array	easy
206	reverse link list	Java - recursive Java - Iterative C++ recursive C++ stack (C++ iterative)	easy
207	course schedule	Java - DFS Java - DFS with edge classification	medium
208	Implement Trie	Java	medium
209	minimum size subarray sum	Java - O(n^2) Java - O(n) Java -O(nlogn)	medium
210	course schedule II	Java - topological sort	medium
211	add and search word - data structure	Java - Trie	medium
212	word search II	Java	hard
213	house robber II	Java - DP	medium
214	shortest palindrome	Java - O(n^2) Java - O(1/4*n^2)	hard
215	kth largest element in an array	Java - quick selection Java - sort Java - priority queue	medium
216	combination sum III	Java - Backtracking	medium
217	Contains Duplicate	C++ Java - hash set Java - insertion sort	easy
219	contains duplicate II	Java	easy
221	maximum square	Java Java - Better DP	medium
222	count complete tree nodes	Java - recursive	medium
223	rectangle area	Java	easy
224	basic calculator	Java	hard
225	implement stack using queue	Java - two queues push O(1) pop O(n) Java - two queue push O(n) pop O(1) Java - one queue	easy
226	Invert Binary Tree	C++ Java - recursive Java - Iterative C - recursive	easy
227	basic calculator II	Java	medium
228	summary ranges	Java	easy
229	majority element II	Java	medium
230	kth element in BST	Java - recursive Java - iterative Java - to array	medium
231	Power Of Two	C++ Java	easy
232	implement queue using stack	Java - two stacks I Java - two stacks II Java - one stack	easy
233	number of digit one	Java	hard
234	palindrome linked list	Java	easy
235	Lowest Common Ancestor of a Binary Search Tree	C++ Java - recursive Java - iterative	easy
236	Lowest ancestor of a binary tree	Java - recursive O(n) Java - iterative	medium
237	Delete Node in a Linked List	C++ Java	easy
238	product Array Except Itself	Java - time O(n) space O(1) C++	medium
239	sliding window maximum	Java - brute force Java - deque	hard
240	search 2D matrix	Java - O(m*log(n)) Java - O(m+n)	easy
241	different wayts to add parenthese	Java Java - divide and conqure with memo	medium
242	valid anagram	Java - cheating - space O(1) Java - space O(1) Java - space O(26) - array C++	easy
243	shortest word distance	Java - HashMap Java - Array	easy
244	shortest word distance II	Java	medium
245	shortest word distance III	Java - O(n)	medium
246	strobogrammatic number	Java - HashMap Java - Array	easy
247	strobogrammatic number II	Java - bfs Java - dfs	medium
249	group shifted strings	Java - hashMap Java - more comlicated	medium
250	count uni value trees	Java - Post Order	medium
251	flattern 2D vector	Java - use pointer Java - use iterator	medium
252	meeting rooms	Java - quick sort Java - comparator	easy
253	meeting room II	Java	medium
256	paint house	Java - DP	easy
257	binary tree path	Java - Recursive Java - recursive with memorization Java - backtracking	easy
258	Add Digits	C++ Java - iterative Java recursive Java digit root O(1) C	easy
259	3 sum smaller	Java - brute force Java - O(n^2) Java - O(n^2* log(n)
260	single number III	Java - space O(1) ugly Java - space O(1) pretty C++	medium
263	ugly number	Java C++	easy
264	ugly number II	Java - DP	medium
266	palindrome permutation	Java - HashSet	easy
268	missing number	Java - time O(n), space O(1) Java - better coding style Java - bitwise C++	medium
269	alien dictionary	Java - Kahn's algo topo sort	hard
270	closest binary search tree	Java - recursive Java - iterative	easy
271	encode decode strings	Java - with deliminator	medium
274	H-index	Java - O(n) Java - O(nlogn)	medium
275	H-Index II	Java - O(log(n))	medium
276	paint fence	Java - recursive Java - DP	easy
277	find celebrity	Java - brute force O(n^2) Java - O(3n) Java - O(2n)	medium
278	first bad version	Java	easy
279	perfect square	Java - DP Java - BFS	medium
280	wiggle sort	java - O(n) greedy	medium
281	zigzag iterator	Java - Queue	medium
282	Expression Add Operator	Java	hard
283	move zeros	Java O(n) Java - O(n) better sol C++	easy
284	peeking iterator	Java	medium
287	find the duplicate number	Java - bloom filter Java - invert in place Java - tortoise and hare	hard
288	unique word abbr	Java - HashMap	medium
289	game of life	Java O(n) Java O(1) Java O(1)	medium
290	word pattern	Java	easy
292	Nim Game	Java C++ C	easy
293	flip game	Java - O(n)	easy
295	find medium from data stream	Java - binary search Java - priority queue	hard
297	Serialize And Deserialize Tree	Java - DFS preorder Java - BFS	hard
299	bulls and cows	Java	easy
300	Longest Increasing Subsequence	Java Java - brute force	meidum
301	remove invalid parentheses	Java -BFS Java - DFS	hard
302	smallest rectangle enclosing black pixels	Java - DFS	hard
303	range sum query	Java	easy
304	Range Sum Query 2D - Immutable	Java - DP	medium
306	additive number	Java	hard
307	range sum query mutable	Java - Binary Index Tree	medium
308	range sum query - 2D mutable	Java - Binary Index Tree	hard
312	burst ballons	Java - DP	hard
313	super ugly number	Java	medium
314	binary tree vertical order traversal	Java	medium
315	count of smaller num after self	Java	hard
316	remove dup letters	Java -array Java - hashMap	hard
318	max product of the word length	Java	medium
319	bulb switcher	Java - solution 1 slow Java - solution 2 fast	medium
320	generalized abbr	Java - HashSet Java - List Java - DFS	medium
322	coin change	Java - DP	medium
323	number of connected compnents in an undirected graph	Java - bfs Java - union find	medium
325	maximum size subarray sum equals k	Java - O(n)	medium
326	power of three	Java - recur Java - iter C++	easy
328	odd even linked list	Java solution C++	easy
329	longest path in matrix	Java	hard
330	patching array	Java Java - simplified
331	verify preorder serialization of a binary tree	Java - node count Java - stack	medium
332	recontruct itinerary	Java	medium
334	increasing triplet subsequence	Java- O(n)	medium
336	Palindrome Pairs	Java - O(n^2*m) Java - Trie	hard
337	house robber III	Java C	medium
338	count bits	Java - O(32 * num) Java - O(1) Java - better O(1) C	medium
339	nested list weight sum	Java - recursive	easy
341	flatten nested list iterator	Java Java - lazy initialization	medium
342	power of four	Java - 1 Java - 2 Java - 3 C	easy
343	integer break	Java C	medium
344	reverse string	Java C	easy
345	reverse vowels of str	Java C	easy
346	moving average from data stream	Java - solution	easy
347	top k frequent elements	Java - Priority Queue	medium
348	design tic tac toe	Java - O(n) Java - O(1)	medium
349	intersection of two arrays	Java - sort and compare Java - BitSet Java - sort one and hashset	easy
350	intersection of two arrays	Java - two pointer	easy
351	android unlock pattern	Java - backtracking	medium
352	data stream as disjoint intervals	Java	hard
353	design snake game	Java - 2D Java - 1D	medium
357	count numbers with unique digits	Java - DP	medium
359	logger ratelimiter	Java - HashMap Java - Queue	easy
362	design hit counter	Java - Queue Java - Array	medium
366	find leaves of binary tree	Java - backtracking Java - remove node	medium
367	valid perfect square	Java - binary search	medium
368	largest divisible subset	Java Java - optimized	medium
369	plus one linked list	Java - reverse Java - recursive Java - two pointer	medium
370	range addition	Java - hashMap Java - range caching	medium
371	sum of integer	Java - Solution 1 Java - better Solution	easy
375	guess num high low II	Java - recursive Java - recursive with memo Java - better recursive with memo	medium
377	combination sum IV	Java - recursive Java - recursive with memo Java - DP	medium
379	design phone diretory	Java	medium
380	insert delete getRandom O(1)	Java	medium
383	ransom note	Java - O(n)	easy
387	first unique char in string	Java - HashSet Java - O(n) array Java - O(n) concise	easy
388	longest absolute file path	Java - Stack	medium
389	find the difference	Java	easy
393	UTF-8 validation	Java	medium
396	rotate func	java - n^2 Java - O(n)	easy
397	integer replacement	Java - 1	medium
401	binary watch	Java	easy
402	remove k digits	Java - Stack	medium
404	sum of leaves	Java - recursive	medium
405	convert number to hex	Java - 1 Java - 2	easy
408	validate word abbr	Java	easy
409	longest palindrome	Java - hashMap Java - array	easy
410	split array largest sum	Java - binary search	hard
415	add string	Java	easy
416	partition equal subset sum	Java - DP	medium
422	valid word square	Java - space O(n) Java - space O(1)	easy
439	ternary expression parser	Java - use str replace Java - use stack Java - dfs	medium
448	Find All Numbers Disappeared in an Array	Java - O(n) Java - O(1)	easy
445	add two numbers II	Java - reverse list Java - stack	medium
450	delete node from bst	Java - recursive	medium
451	sort char by freq	Java - PriorityQueue	medium
456	find 132 pattern	Java - O(n^3) Java - O(n^2) Java - O(n)	medium
459	Repeated Substring Pattern	Java	easy
461	hamming distance	Java - Solution 1 Java - alternative	easy
463	island perimeter	Java	easy
476	number complement	Java - Solution 1 Java - Java API	easy
482	license key formatting	Java - Stack Java - no stack	medium
490	the maze	Java - DFS Java - BFS	medium
494	target sum	Java - brute force Java - brute force with memo Java - DP Java - DP O(n)	medium
496	next greater element	Java - O(m*n) Java - O(m*n) Java - O(n)	easy
500	keyboard word	Java	easy
503	next greater element II	brute force - O(n^2) O(2n) brute force O(n^2) O(n) Java - Stack	medium
515	find largest in each tree row	Java - BFS Java - DFS	medium
513	find bottom left tree	Java - BFS Java - Recursive	medium
520	detect capital	Java Java use char-> int	easy
529	mine sweeper	Java	medium
530	find minimum abs diff in bst	Java - traversal & sort Java - inorder	easy
531	lonely pixel I	Java - O(n) Java - O(1)	medium
532	k diff pairs in array	Java - HashMap	easy
536	construct binary tree from str	Java	medium
537	multiply complex num	Java	easy
538	convert bst to greater tree	Java	medium
543	diameter of binary tree	Java - recursive	easy
544	Output Contest Matches	Java - Solution	medium
544	Output Contest Matches	Java - Solution	medium
545	boundary of binary tree	Java - Solution	hard
547	friend circle	Java - union find	medium
551	student attendance record I	Java - O(n) Java - regex	easy
553	optimal division	Java - Math	medium
560	subarray sum equals k	Java - brute force Java - store the sum O(n^2) O(n) Java - O(n^2) O(1) Java - HashMap	medium
561	array partition I	Java - Quick Sort Java - array	easy
570	manager with at least 5 direct reporters	Mysql	easy
572	subtree of another tree	Java - preorder Java - dfs	easy
581	shortest unsorted continuous subarray	Java - space O(n) Java - span O(1)	easy
582	kill process	Java - DFS recursive Java - DFS stack Java - BFS	medium
583	delete operation for 2 strings	Java - longest commen str Java - longest with memorization Java - DP longest common Java - DP	medium
595	big countries	SQL	easy
604	design compressed string iterator	Java - O(m) Java - O(#distint char)	medium
605	can place flower	Java - DP	easy
606	construct str from binary tree	Java - recursive Java - iterative Java - recursive another	medium
609	find duplicated file in the system	Java - HashMap	medium
611	valid triangle number	Java - O(n^2)	medium
613	shortest distance in line	MySQL	easy
617	merge two binary tree	Java - recursive Java - iterative	easy
627	swap salary	MySQL	easy
635	design log storage system	Java - list Java - TreeMap	medium
637	average of levels in binary tree	Java - BFS	easy
639	decode ways II	Java - recursive Java - DP	hard
640	solve the equation	Java - regex	medium
643	max average subarray I	Java	easy
645	set mismatch	Java - Set Java - flip array ele	easy
646	max len of pair chain	Java - sort	medium
653	two sum IV	Java - hashset Java - traversal	easy
657	Judge Route Circle	Java	easy
661	image smoother	Java	easy
662	max width of binary tree	Java - BFS	medium
663	Equal Tree Partition	Java - dfs	medium
669	trim bst	Java - recursive	easy