【Leetcode】75. Sort Colors

Given an array with n objects colored red, white or blue, sort them in-placeso that objects of the same color are adjacent, with the colors in the order red, white and blue.

Here, we will use the integers 0, 1, and 2 to represent the color red, white, and blue respectively.

Note:You are not suppose to use the library's sort function for this problem.

1 可以使用counting sort，先遍历一遍array，找出0，1，2的个数，然后再去overwrite array，所以是一个two pass的过程

2 使用partition方法，采用三个指针，red，white，blue，red=white=0，blue=n-1，循环条件是white<=blue，当nums[white]=2时，和nums[n-1]交换，同时blue-1，这里是为了将blue的都放在最后面去；当nums[white]=1的时候，不需要交换任何的，直接把white指针加1就行；当nums[white]=0的时候，和nums[red]进行交换，同时把white和red的指针都加1