| 075|[Sort Colors](https://github.com/interviewcoder/leetcode/blob/master/src/_075_SortColors/Solution.java)| Medium|`Array``Two Pointers``Sort`||

| 076|[Minimum Window Substring](https://github.com/interviewcoder/leetcode/blob/master/src/_076_MinimumWindowSubstring/Solution.java)| Hard|`Hash Table``Two Pointers``String`|`!`**[substring/sublist]** hashing + two pointers -> O(N)|

| 077|[Combinations](https://github.com/interviewcoder/leetcode/blob/master/src/_077_Combinations/Solution.java)| Medium|`Backtracking`||

| 078|[Subsets](https://github.com/interviewcoder/leetcode/blob/master/src/_078_Subsets/Solution.java)| Medium|`Array``Backtracking``Bit Manipulation`|backtrack template (see[README](https://github.com/interviewcoder/leetcode/blob/master/src/_078_Subsets))|

| 078|[Subsets](https://github.com/interviewcoder/leetcode/blob/master/src/_078_Subsets/)| Medium|`Array``Backtracking``Bit Manipulation`| see[README](https://github.com/interviewcoder/leetcode/blob/master/src/_078_Subsets)|

| 079|[Word Search](https://github.com/interviewcoder/leetcode/blob/master/src/_079_WordSearch/Solution.java)| Medium|`Array``Backtracking`||

| 080|[Remove Duplicates from Sorted Array II](https://github.com/interviewcoder/leetcode/blob/master/src/_080_RemoveDuplicatesFromSortedArrayII/)| Medium|`Array``Two Pointers`| from 2 to k (general)|

| 081|[Search in Rotated Sorted Array II](https://github.com/interviewcoder/leetcode/blob/master/src/_081_SearchInRotatedSortedArrayII/Solution.java)| Medium|`Array``Binary Search`||

| 123|[Best Time to Buy and Sell Stock III](https://github.com/interviewcoder/leetcode/blob/master/src/_123_BestTimeToBuyAndSellStockIII/Solution.java)| Hard|`Array``Dynamic Programming`||

| 124|[Binary Tree Maximum Path Sum](https://github.com/interviewcoder/leetcode/blob/master/src/_124_BinaryTreeMaximumPathSum/)| Hard|`Tree``Depth-first Search`| post-order, local and global ,[README](https://github.com/interviewcoder/leetcode/tree/master/src/_124_BinaryTreeMaximumPathSum)|

| 125|[Valid Palindrome](https://github.com/interviewcoder/leetcode/blob/master/src/_125_ValidPalindrome/Solution.java)| Easy|`Two Pointers``String`||

| 126|[Word Ladder II](https://github.com/interviewcoder/leetcode/blob/master/src/_126_WordLadderII/Solution.java)| Hard|`Array``Backtracking``Breadth-first Search``String`||

| 127|[Word Ladder](https://github.com/interviewcoder/leetcode/blob/master/src/_127_WordLadder/Solution.java)| Medium|`Breadth-first Search`||

| 126|[Word Ladder II](https://github.com/interviewcoder/leetcode/blob/master/src/_126_WordLadderII/)| Hard|`Array``Backtracking``Breadth-first Search``String`||

| 127|[Word Ladder](https://github.com/interviewcoder/leetcode/blob/master/src/_127_WordLadder/)| Medium|`Breadth-first Search`||

| 128|[Longest Consecutive Sequence](https://github.com/interviewcoder/leetcode/blob/master/src/_128_LongestConsecutiveSequence/Solution.java)| Hard|`Array`||

| 129|[Sum Root to Leaf Numbers](https://github.com/interviewcoder/leetcode/blob/master/src/_129_SumRootToLeafNumbers/Solution.java)| Medium|`Tree``Depth-first Search`||

| 130|[Surrounded Regions](https://github.com/interviewcoder/leetcode/blob/master/src/_130_SurroundedRegions/Solution.java)| Medium|`Breadth-first Search`||

| 160|[Intersection of Two Linked Lists](https://github.com/interviewcoder/leetcode/blob/master/src/_160_IntersectionOfTwoLinkedLists/Solution.java)| Easy|`Linked List`| two pointers on two lists; combine long and short lists into one|

| 161|[One Edit Distance](https://github.com/interviewcoder/leetcode/blob/master/src/_161_OneEditDistance/)| Medium|`String`||

| 162|[Find Peak Element](https://github.com/interviewcoder/leetcode/blob/master/src/_162_FindPeakElement/Solution.java)| Medium|`Array``Binary Search`||

| 163|[Missing Ranges](https://github.com/interviewcoder/leetcode/blob/master/src/_163_MissingRanges/Solution.java)| Medium|`Array`||

| 164|[Maximum Gap](https://github.com/interviewcoder/leetcode/blob/master/src/_164_MaximumGap/Solution.java)| Hard|`Sort`||

| 165|[Compare Version Numbers](https://github.com/interviewcoder/leetcode/blob/master/src/_165_CompareVersionNumbers/Solution.java)| Easy|`String`||

| 166|[Fraction to Recurring Decimal](https://github.com/interviewcoder/leetcode/blob/master/src/_166_FractionToRecurringDecimal/Solution.java)| Medium|`Hash Table``Math`||

| 282| Expression Add Operators| Hard|`Divide and Conquer`||

| 283|[Move Zeroes](https://github.com/interviewcoder/leetcode/blob/master/src/_283_MoveZeroes/Solution.java)| Easy|`Array``Two Pointers`||

| 284| Peeking Iterator| Medium|`Design`||

| 285|[Inorder Successor in BST](https://github.com/interviewcoder/leetcode/tree/master/src/_285_InorderSuccessorInBST)| Medium|`Tree`||

| 286|[Walls And Gates](https://github.com/interviewcoder/leetcode/tree/master/src/_286_WallsAndGates)| Medium|`BFS`||

| 287| Find the Duplicate Number| Hard|`Array``Two Pointers``Binary Search`||

0.[Partition](https://github.com/interviewcoder/leetcode/blob/master/doc/category_partition.md)

0.[Subarray Problem](https://github.com/interviewcoder/leetcode/blob/master/doc/category_subarray.md)

0.[Subarray Problem](https://github.com/interviewcoder/leetcode/blob/master/doc/category_subarray.md)

0.[Similar Problems](https://github.com/interviewcoder/leetcode/blob/master/doc/category_similar.md)

| 1 sequence :`max`/min(*)| 032|[Longest Valid Parentheses](https://github.com/interviewcoder/leetcode/tree/master/src/_032_LongestValidParentheses)| global V.S. local|

| 1 sequence :`max`/min(*)| 121| Best Time to Buy and Sell Stock||

| 1 sequence :`max`/min(*)| 123| Best Time to Buy and Sell Stock III||

| 1 sequence : count(*)| 091| Decode Ways||

| 1 sequence : count(*)| 091|[Decode Ways](https://github.com/interviewcoder/leetcode/tree/master/src/_091_DecodeWays)||

| 1 sequence : all(*)| 140| Word Break II: dp + backtracking||

| Type| #| Description|

| ---------------------:|:---:| ------------|

| heap V.S. merge| 023|https://github.com/interviewcoder/leetcode/tree/master/src/_023_MergeKSortedLists|

| heap V.S. merge| 218|[The Skyline Problem](https://github.com/interviewcoder/leetcode/tree/master/src/_218_TheSkylineProblem)|

| comsume cache| 158|[Read N Characteres Given Read4 II](https://github.com/interviewcoder/leetcode/blob/master/src/_158_ReadNCharactersGivenRead4II/Solution.java)|

| comsume cache| 284|[Peeking Iterator](https://github.com/interviewcoder/leetcode/blob/master/src/_284_PeekingIterator/Solution.java)|

|`continuous`|| continuous subarray||

| Two Pointers : maxsize| 005|[Longest Palindromic Substring](https://github.com/interviewcoder/leetcode/blob/master/src/_005_LongestPalindromicSubstring/Solution.java)| O(N^3) -> O(N^2)|

| Two Pointers : maxsize| 003|[Longest Substring Without Repeating Characters](https://github.com/interviewcoder/leetcode/blob/master/src/_003_LongestSubstringWithoutRepeatingCharacters/Solution.java)| O(N^3) -> O(N) + O(N) hash|

| Two Pointers : maxsize| 159|[Longest Substring With At Most Two Distinct Characters](https://github.com/interviewcoder/leetcode/blob/master/src/_159_LongestSubstringWithAtMostTwoDistinctCharacters/Solution.java)| O(N^3) -> O(N) + O(2) hash|

| Two Pointers : minsize| 076|[Minimum Window Substring](https://github.com/interviewcoder/leetcode/tree/master/src/_076_MinimumWindowSubstring)| O(N^3) -> O(N) + O(N) hash|

| Two Pointers : minsize| 209|[Minimum Size Subarray Sum](https://github.com/interviewcoder/leetcode/tree/master/src/_209_MinimumSizeSubarraySum)| O(N^3) -> O(N)|

| Two Pointers : accumulative area| 042|[Trapping Rain Water](https://github.com/interviewcoder/leetcode/blob/master/src/_042_TrappingRainWater/Solution.java)||

1. Iterative

1.1) Use stack to simulate recursive DFS

1.2) BFS with queue

2. Recursive

2. Recursive (DFS)

2.1) visit root first = pre-order = top-down = preorder traversal

2.2) visit root last = post-order = bottom-up = divide and conquer

[Lowest Common Ancestor of Binary Tree]

2.3) visit root middle = in-order traversal

| level traversal| 103| Binary Tree Zigzag Level Order Traversal|

|*`post`*-order| 124|[Binary Tree Maximum Path Sum](https://github.com/interviewcoder/leetcode/tree/master/src/_124_BinaryTreeMaximumPathSum)|

|*`post`*-order| 129|[Sum Root To Leaf Numbers](https://github.com/interviewcoder/leetcode/blob/master/src/_129_SumRootToLeafNumbers/Solution.java)|

|*`post`*-order| 104|[Maximum Depth Of Binary Tree](https://github.com/interviewcoder/leetcode/tree/master/src/_104_MaximumDepthOfBinaryTree)|

|*`post`*-order| 236|[Lowest Common Ancestor of Binary Tree](https://github.com/interviewcoder/leetcode/tree/master/src/_236_LowestCommonAncestorOfABinaryTree)|

| inorder + prev| 098|[Validate Binary Search Tree](https://github.com/interviewcoder/leetcode/tree/master/src/_098_ValidateBinarySearchTree)|

| inorder + prev| 099|[Recover Binary Search Tree](https://github.com/interviewcoder/leetcode/tree/master/src/_099_RecoverBinarySearchTree)|

| inorder + prune| Lint|[Search Range In Binary Search Tree](https://github.com/interviewcoder/lintcode/blob/master/03_binarytree%26divideconquer/_02_SearchRangeInBinarySearchTree/Solution.java)|

| inorder + iterative| 173|[Binary Search Tree Iterator](https://github.com/interviewcoder/leetcode/blob/master/src/_173_BinarySearchTreeIterator/Solution.java)|

|`inorder` + prev| 098|[Validate Binary Search Tree](https://github.com/interviewcoder/leetcode/tree/master/src/_098_ValidateBinarySearchTree)|

|`inorder` + prev| 099|[Recover Binary Search Tree](https://github.com/interviewcoder/leetcode/tree/master/src/_099_RecoverBinarySearchTree)|

|`inorder` + prune| Lint|[Search Range In Binary Search Tree](https://github.com/interviewcoder/lintcode/blob/master/03_binarytree%26divideconquer/_02_SearchRangeInBinarySearchTree/Solution.java)|

|`inorder` + iterative| 173|[Binary Search Tree Iterator](https://github.com/interviewcoder/leetcode/blob/master/src/_173_BinarySearchTreeIterator/Solution.java)|