LeetCode--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?

思路：双指针法。
这个题直接的解法是计数排序，两次遍历，第一次记录三个数字分别有多少个，第二次遍历分别按个数写0,1,2.
巧妙的解法是采用双指针法，头指针是0，尾指针是2，一次遍历，循环检查，当扫描到2的时候和尾指针交换，尾指针-1，当扫描到0的时候和头指针交换，头指针+1.

class Solution {public:    void sortColors(vector<int>& nums) {        int second=nums.size()-1,zero=0;        for(int i=0;i<=second;i++){            while(nums[i]==2&&i<second) swap(nums[i],nums[second--]);            while(nums[i]==0&&i>zero) swap(nums[i],nums[zero++]);        }    }};