75 Sort Colors

链接：https://leetcode.com/tag/two-pointers/
问题描述：
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?
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其实就是一个排序的问题，只不过里面数字只有三个。首先想到sort函数，一行就可以解决。

class Solution {
public:
    void sortColors(vector<int>& nums) {
     sort(nums.begin(),nums.end());
    }
};

既然数组里面只有三个数字，那么可以定义三个指针，p0表示最后一个0所在的位置，p1表示最后一个1所在的位置，p2表示最后一个2所在的位置。遍历数组，不断更新这三个指针就可以了。

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int p0=0,p1=0,p2=0;
        for(int i=0;i<nums.size();i++)
        {
          if(nums[i]==0)
          {
            nums[p2++]=2;
            nums[p1++]=1;
            nums[p0++]=0;           
          }
          else if(nums[i]==1)
          {
            nums[p2++]=2;
            nums[p1++]=1;
          }
          else if(nums[i]==2)
            nums[p2++]=2;
        }
    }
};