|236|[Lowest Common Ancestor of a Binary Tree](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/)|[Solution](../master/src/main/java/com/stevesun/solutions/_236.java)| O(n)|O(h) | Medium| DFS

|235|[Lowest Common Ancestor of a Binary Search Tree](https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-search-tree/)|[Solution](../master/src/main/java/com/stevesun/solutions/_235.java)| O(h)|O(1) | Easy| DFS

|233|[Number of Digit One](https://leetcode.com/problems/number-of-digit-one/)|[Solution](../master/src/main/java/com/stevesun/solutions/NumberofDigitOne.java)| O(n)|O(1) | Hard| Math

|229|[Majority Element II](https://leetcode.com/problems/majority-element-ii/)|[Solution](../master/src/main/java/com/stevesun/solutions/MajorityElementII.java)| O(n)|O(n) | Medium|

|228|[Summary Ranges](https://leetcode.com/problems/summary-ranges/)|[Solution](../master/src/main/java/com/stevesun/solutions/SummaryRanges.java)| O(n)|O(1) | Medium| Array

|226|[Invert Binary Tree](https://leetcode.com/problems/invert-binary-tree/)|[Solution](../master/src/main/java/com/stevesun/solutions/InvertBinaryTree.java)| O(n)|O(h) | Easy| DFS, recursion

|201|[Bitwise AND of Numbers Range](https://leetcode.com/problems/bitwise-and-of-numbers-range/)|[Solution](../master/src/main/java/com/stevesun/solutions/_201.java)| O(min(m,n))|O(1) | Medium | Bit Manipulation

|200|[Number of Islands](https://leetcode.com/problems/number-of-islands/)|[Union Find](../master/src/main/java/com/stevesun/solutions/_200UnionFind.java) [DFS](../master/MEDIUM/src/medium/_200DFS.java)| O(m*n)|O(m*n) | Medium| Union Find, DFS

|198|[House Robber](https://leetcode.com/problems/house-robber/)|[Solution](../master/src/main/java/com/stevesun/solutions/HouseRobber.java)| O(n)|O(n)| Easy | DP

|190|[Reverse Bits](https://leetcode.com/problems/reverse-bits/)|[Solution](../master/src/main/java/com/stevesun/solutions/ReverseBits.java)| O(n)|O(1)| Easy | Bit Manipulation

|189|[Rotate Array](https://leetcode.com/problems/rotate-array/)|[Solution](../master/src/main/java/com/stevesun/solutions/_189.java)| O(n)|O(n), could be optimized to O(1) | Easy

|187|[Repeated DNA Sequences](https://leetcode.com/problems/repeated-dna-sequences/)|[Solution](../master/src/main/java/com/stevesun/solutions/_187.java)| O(n)|O(n) | Medium

|163|[Missing Ranges](https://leetcode.com/problems/missing-ranges/)|[Solution](../master/src/main/java/com/stevesun/solutions/MissingRanges.java) | O(n) |O(1) | |

|162|[Find Peak Element](https://leetcode.com/problems/find-peak-element/)|[Solution](../master/src/main/java/com/stevesun/solutions/FindPeakElement.java) | O(1) |O(logn)/O(n) | Binary Search|

|161|[One Edit Distance](https://leetcode.com/problems/one-edit-distance/)|[Solution](../master/src/main/java/com/stevesun/solutions/OneEditDistance.java) | O(n) |O(1) | |

|159|[Longest Substring with At Most Two Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/)|[Solution](../master/src/main/java/com/stevesun/solutions/LongestSubstringwithAtMostTwoDistinctCharacters.java)| O(n)|O(1) | Hard| String, Sliding Window

|158|[Read N Characters Given Read4 II - Call multiple times](https://leetcode.com/problems/read-n-characters-given-read4-ii-call-multiple-times/)|[Solution](../master/src/main/java/com/stevesun/solutions/ReadNCharactersGivenRead4IICallMultipleTimes.java)| O(n)|O(1) | Hard|

|157|[Read N Characters Given Read4](https://leetcode.com/problems/read-n-characters-given-read4/)|[Solution](../master/src/main/java/com/stevesun/solutions/ReadNCharactersGivenRead4.java)| O(n)|O(1) | Easy|

|105|[Construct Binary Tree from Preorder and Inorder Traversal](https://leetcode.com/problems/construct-binary-tree-from-preorder-and-inorder-traversal/)|[Solution](../master/src/main/java/com/stevesun/solutions/_105.java)| O(n)|O(n) | Medium| Recursion, Tree

|104|[Maximum Depth of Binary Tree](https://leetcode.com/problems/maximum-depth-of-binary-tree/)|[Solution](../master/src/main/java/com/stevesun/solutions/MaximumDepthOfBinaryTree.java)| O(n)|O(h) | Easy| DFS

|103|[Binary Tree Zigzag Level Order Traversal](https://leetcode.com/problems/binary-tree-zigzag-level-order-traversal/)|[Solution](../master/src/main/java/com/stevesun/solutions/_103.java)| O(n)|O(h) | Medium| BFS,DFS

|99|[Recover Binary Search Tree](https://leetcode.com/problems/recover-binary-search-tree/)|[Solution](../master/src/main/java/com/stevesun/solutions/RecoverBinarySearchTree.java) | O(?) | O(?) | Hard |

|98|[Validate Binary Search Tree](https://leetcode.com/problems/validate-binary-search-tree/)|[Solution](../master/src/main/java/com/stevesun/solutions/_98.java) | O(n) | O(h) | Medium | DFS/Recursion

|97|[Interleaving String](https://leetcode.com/problems/interleaving-string/)|[Solution](../master/src/main/java/com/stevesun/solutions/InterleavingString.java)| O(?)|O(?) | Hard| DP

|74|[Search a 2D Matrix](https://leetcode.com/problems/search-a-2d-matrix/)|[Solution](../master/src/main/java/com/stevesun/solutions/Searcha2DMatrix.java)|O(logn)|O(1)|Medium| Binary Search

|73|[Set Matrix Zeroes](https://leetcode.com/problems/set-matrix-zeroes/)|[Solution](../master/src/main/java/com/stevesun/solutions/_73.java)|O(mn)|O(mn)|Medium|

|72|[Edit Distance](https://leetcode.com/problems/edit-distance/)|[Solution](../master/src/main/java/com/stevesun/solutions/EditDistance.java)|O(m*n)|O(m+n)|Hard|

|70|[Climbing Stairs](https://leetcode.com/problems/climbing-stairs/)|[Solution](../master/src/main/java/com/stevesun/solutions/ClimbingStairs.java)|O(n)|O(n)|Easy| DP

|69|[Sqrt(x)](https://leetcode.com/problems/sqrtx/)|[Solution](../master/src/main/java/com/stevesun/solutions/Sqrt.java)|O(logn)|O(1)|Easy|

|68|[Text Justification](https://leetcode.com/problems/text-justification/)|[Solution](../master/src/main/java/com/stevesun/solutions/TextJustification.java)|O(n)|O(1)|Hard|

|36|[Valid Sudoku](https://leetcode.com/problems/valid-sudoku/)|[Solution](../master/src/main/java/com/stevesun/solutions/ValidSudoku.java)|O(?)|O(?)|Medium|

|35|[Search Insert Position](https://leetcode.com/problems/search-insert-position/)|[Solution](../master/src/main/java/com/stevesun/solutions/SearchInsertPosition.java)|O(n)|O(1)|Medium|Array

|34|[Search for a Range](https://leetcode.com/problems/search-for-a-range/)|[Solution](../master/src/main/java/com/stevesun/solutions/SearchForARange.java)|O(logn)|O(1)|Medium|Array, Binary Search

|32|[Longest Valid Parentheses](https://leetcode.com/problems/longest-valid-parentheses/)|[Solution](../master/src/main/java/com/stevesun/solutions/LongestValidParentheses.java)|O(n)|O(n)|Hard|Stack, DP

|31|[Next Permutation](https://leetcode.com/problems/next-permutation)|[Solution](../master/src/main/java/com/stevesun/solutions/NextPermutation.java)|O(n)|O(1)|Medium|Array

|30|[Substring with Concatenation of All Words](https://leetcode.com/problems/substring-with-concatenation-of-all-words/)|[Solution](../master/src/main/java/com/stevesun/solutions/SubstringwithConcatenationofAllWords.java)|O(n^2)|O(n)|Hard| HashMap

|29|[Divide Two Integers](https://leetcode.com/problems/divide-two-integers/)|[Solution](../master/src/main/java/com/stevesun/solutions/DivideTwoIntegers.java)|O(?)|O(?)|Medium|

|28|[Implement strStr()](https://leetcode.com/problems/implement-strstr/)|[Solution](../master/src/main/java/com/stevesun/solutions/ImplementStrStr.java)|O(n)|O(1)|Easy| String

|27|[Remove Element](https://leetcode.com/problems/remove-element/)|[Solution](../master/src/main/java/com/stevesun/solutions/RemoveElement.java)|O(n)|O(1)| Easy |

|25|[Reverse Nodes in k-Group](https://leetcode.com/problems/reverse-nodes-in-k-group/)|[Solution](../master/src/main/java/com/stevesun/solutions/ReverseNodesinkGroup.java)|O(n)|O(1)| Hard | Recursion, LinkedList

|23|[Merge k Sorted Lists](https://leetcode.com/problems/merge-k-sorted-lists/)|[Solution](../master/src/main/java/com/stevesun/solutions/MergeKSortedList.java)|O(n*logk)|O(logk)|Hard|Heap

|22|[Generate Parentheses](https://leetcode.com/problems/generate-parentheses/)|[Solution](../master/src/main/java/com/stevesun/solutions/GenerateParentheses.java)|TBD|O(n)|Medium|Backtracking

|21|[Merge Two Sorted Lists](https://leetcode.com/problems/merge-two-sorted-lists/)|[Solution](../master/src/main/java/com/stevesun/solutions/_21.java)|O(n)|O(h)|Easy| Recursion

|8|[String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/)|[Solution](../master/src/main/java/com/stevesun/solutions/_8.java)| O(n) | O(1) | Medium

|7|[Reverse Integer](https://leetcode.com/problems/reverse-integer/)|[Solution](../master/src/main/java/com/stevesun/solutions/ReverseInteger.java) | O(1) | O(1) | Easy |

|6|[ZigZag Conversion](https://leetcode.com/problems/zigzag-conversion/)|[Solution](../master/src/main/java/com/stevesun/solutions/_6.java) | O(n) | O(n) | Easy |

|4|[Median of Two Sorted Arrays](https://leetcode.com/problems/median-of-two-sorted-arrays/)|[Solution](../master/src/main/java/com/stevesun/solutions/MedianofTwoSortedArrays.java) | ? | ? | Hard | Divide and Conquer

|3|[Longest Substring Without Repeating Characters](https://leetcode.com/problems/longest-substring-without-repeating-characters/)|[Solution](../master/src/main/java/com/stevesun/solutions/LongestSubstringWithoutRepeatingCharacters.java) | O(n) | O(1) | Medium | HashMap, Sliding Window

|2|[Add Two Numbers](https://leetcode.com/problems/add-two-numbers/)|[Solution](../master/src/main/java/com/stevesun/solutions/_2.java) | O(max(m,n)) | O(1) | Medium | LinkedList

public boolean isPowerOfTwo(int n) {

//after writing out the binary representation of some numbers: 1,2,4,8,16,32, you can easily figure out that

//every number that is power of two has only one bit that is 1

return n> 0 && (n&(n-1)) == 0;

}

public List<List<Integer>> levelOrder(TreeNode root) {

List<List<Integer>> result = new ArrayList<List<Integer>>();

if(root == null) return result;