Sort Colors

Sort Colors

Given an array with n objects colored red, white or blue, sort them so 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.

click to show follow up.

Follow up:
A rather straight forward solution is a two-pass algorithm using counting sort.
First, iterate the array counting number of 0's, 1's, and 2's, then overwrite array with total number of 0's, then 1's and followed by 2's.

Could you come up with an one-pass algorithm using only constant space?

解题技巧：

首先，定义两个指针：start = -1，end = n。其次，进行一次遍历，遇到0时，交换给start；遇到2时，交换给end；遇到1不需要处理。

代码：

void Swap(vector<int>& nums, int a, int b){    int tmp;    tmp = nums[a];    nums[a] = nums[b];    nums[b] = tmp;}void sortColors(vector<int>& nums){     int bg = -1, ed = nums.size(), i = 0, tmp;     while(i < nums.size())     {         if(nums[i] == 0)         {             if(bg < i)                 Swap(nums, ++bg, i);             else i++;         }         else if(nums[i] == 2)         {             if(i < ed)                Swap(nums, --ed, i);             else i++;         }         else            i++;     }}