Multiply Strings

Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.
（用非库函数的方式返回两个字符串表示的整数的乘积）

Trapping Rain Water

Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
（收集雨水）

First Missing Positive

Given an unsorted integer array, find the smallest missing positive integer.
（在未排序数组中找到最小的非正数）

Combination Sum II

Given a collection of candidate numbers (candidates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target. Each number in candidates may only be used once in the combination.
（从集合中挑选和为特定值的数字组合，同一元素只可选一次）

Combination Sum

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target. The same repeated number may be chosen from candidates unlimited number of times.
（从集合中挑选和为特定值的数字组合，同一元素可选多次）

Count and Say

读字符串

Sudoku Solver

Write a program to solve a Sudoku puzzle by filling the empty cells.
（求解数独盘）

Valid Sudoku

Determine if a 9x9 Sudoku board is valid. Only the filled cells need to be validated according to the following rules.
（数独盘有效性判断）

Search Insert Position

Given a sorted array and a target value, return the index if the target is found. If not, return the index where it would be if it were inserted in order. You may assume no duplicates in the array.
（在有序数组中检索）

Find First and Last Position of Element in Sorted Array

Given an array of integers nums sorted in ascending order, find the starting and ending position of a given target value. Your algorithm’s runtime complexity must be in the order of O(log n). If the target is not found in the array, return [-1, -1].

（在时间复杂度为O(log n)的前提下在有序数组中检索target的第一次及最后一次）