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# 91. Decode Ways

A message containing letters from A-Z is being encoded to numbers using the following mapping: 'A' -> 1 'B' -> 2 ... 'Z' -> 26 Given an encoded message containing digits, determine the...# 37. Word Search

DFS + Hashmap. Search for every four directions to see if the letter could match the word. If true, go into that direction. Else track back to go to the other directions. We also need a hash map to...# 338. Counting Bits

Given a non negative integer number num. For every numbers i in the range 0 ≤ i ≤ num calculate the number of 1's in their binary representation and return them as an array. Example: For num =...# Linked List Cycle I, II; Find the Duplicate Number

这几道题目都比较类似，都是用快慢指针法(fast, slow pointers)，所以放在一起说好了。**Linked List Cycle**Given a linked list, determine if it has a cycl...

# 73. Set Matrix Zeroes

Given a*m*x

*n*matrix, if an element is 0, set its entire row and column to 0. Do it in place. 由于要求"do it in place"，所以也就是说要求我们只能用O(1)的空间复杂...

# 93. Restore IP Addresses

Given a string containing only digits, restore it by returning all possible valid IP address combinations. For example: Given "25525511135", return ["255.255.11.135", "255.255.111.35"]. (Order ...# 57. Insert Interval

Given a set of non-overlapping intervals, insert a new interval into the intervals (merge if necessary). You may assume that the intervals were initially sorted according to their start times. E...# 56. Merge Intervals

Given a collection of intervals, merge all overlapping intervals. For example, Given [1,3],[2,6],[8,10],[15,18], return [1,6],[8,10],[15,18]. 该题目用Greedy，先对start的时间进行从...# 174. Dungeon Game

The demons had captured the princess (**P**) and imprisoned her in the bottom-right corner of a dungeon. The dungeon consists of M x N rooms laid out in a 2D grid. Our valiant knight (

**K**) was...