## Convert Sorted List to Binary Search Tree

Description LeetCode 109 Given a singly linked list where elements are sorted in ascending order, convert it to a height balanced BST. For this problem, a height-balanced binary tree is defined as a binary tree in which the depth of the two subtrees of every node never differ by more than 1. Example: Given the sorted linked

## First Missing Positive

Description LeetCode 41 Given an unsorted integer array, find the smallest missing positive integer. Example 1: Input: [1,2,0] Output: 3 Example 2: Input: [3,4,-1,1] Output: 2 Example 3: Input: [7,8,9,11,12] Output: 1 Note: Your algorithm should run in O(n) time and uses constant extra space. Analysis From the description, we can find an important clue that the

## Recover Binary Search Tree

Description LeetCode 99 Two elements of a binary search tree (BST) are swapped by mistake. Recover the tree without changing its structure. Example 1: Input: [1,3,null,null,2]   1   /  3   \   2 Output: [3,1,null,null,2]   3   /  1   \   2 Example 2: Input: [3,1,4,null,null,2] 3 / \ 1 4

## Construct all possible BSTs for keys 1 to N

Description LeetCode 95 Given an integer n, generate all structurally unique BST’s (binary search trees) that store values 1 … n. Example: Input: 3 Output: [   [1,null,3,2],   [3,2,null,1],   [3,1,null,null,2],   [2,1,3],   [1,null,2,null,3] ] Explanation: The above output corresponds to the 5 unique BST’s shown below: 1 3 3 2 1 \ / / / \

## Subsets with Duplicates

Description LeetCode 90 Given a collection of integers that might contain duplicates, nums, return all possible subsets (the power set). Note: The solution set must not contain duplicate subsets. Example: Input: [1,2,2] Output: [ [2], [1], [1,2,2], [2,2], [1,2], [] ] Analysis Subset problem has not only one solution. The easiest solution is bit method. Just use

## Scramble String

Description LeetCode 87 Given a string s1, we may represent it as a binary tree by partitioning it to two non-empty substrings recursively. Below is one possible representation of s1 = “great”: great / \ gr eat / \ / \ g r e at / \ a t To scramble the string, we may choose any non-leaf node

## Maximal Rectangle

Description LeetCode 85 Given a 2D binary matrix filled with 0’s and 1’s, find the largest rectangle containing only 1’s and return its area. Example: Input: [ [“1″,”0″,”1″,”0″,”0”], [“1″,”0″,”1″,”1″,”1”], [“1″,”1″,”1″,”1″,”1”], [“1″,”0″,”0″,”1″,”0”] ] Output: 6 Analysis When firstly considering about this problem, the problem seems to be a extension of Largest Rectangle in Histogram. But how can

## Largest Rectangle in Histogram

Description LeetCode 84 Given n non-negative integers representing the histogram’s bar height where the width of each bar is 1, find the area of largest rectangle in the histogram. Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3].   The largest rectangle is shown in the shaded area, which has area = 10 unit.

## Word Search

Description LeetCode 79 Given a 2D board and a word, find if the word exists in the grid. The word can be constructed from letters of sequentially adjacent cell, where “adjacent” cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once. Example: board = [ [‘A’,’B’,’C’,’E’], [‘S’,’F’,’C’,’S’],

## Edit Distance

Description Given two words word1 and word2, find the minimum number of operations required to convert word1 to word2. You have the following 3 operations permitted on a word: Insert a character Delete a character Replace a character Example 1: Input: word1 = “horse”, word2 = “ros” Output: 3 Explanation: horse -> rorse (replace ‘h’ with ‘r’) rorse -> rose (remove