sort

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sort

题目描述
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?

思路很简单只需要一遍遍历,前面为0区间,后面为2区间,当前遍历为i,A[i]==0放入0区间,i加1,0区间加1,A[i]==2,交换,2区间向前移动一个.i则不能变化,因为可能交换为0,若为1则直接i+1跳过.

class Solution {
public:void sortColors(int A[], int n) {if(A==NULL||n<=0)return;int zerobegin=0;int twoend=n-1;int k=0;while(k<=twoend){if(A[k]==0){swap(A[k],A[zerobegin]);++zerobegin;++k;}else if(A[k]==2){swap(A[k],A[twoend]);--twoend;}else++k;}       }
};

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