题目

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.

最简单的方法，先统计出现0、1、2出现的次数，然后直接给数组赋值就ok了

public class Solution {
    public void sortColors(int[] A) {
        int rnum=0;
        int wnum=0;
        int bnum=0;
        for(int i=0;i<A.length;i++){
            if(A[i]==0)
                rnum++;
            else if(A[i]==1)
                wnum++;
            else
                bnum++;
        }
        for(int i=0;i<rnum;i++)
            A[i]=0;
        for(int i=0,j=rnum;i<wnum;i++)
            A[j++]=1;
        for(int i=0,j=rnum+wnum;i<bnum;i++)
            A[j++]=2;
    }
}

另外，因为顺序是0,1,2，可对头尾进行操作

public void sortColors(int[] A){
	int end=A.length-1;
	int start=0;
	for(int i=0;i<=end;i++){
		if(A[i]==0){
			swap(A,i,start);
			start++;
		}
		if(A[i]==2){
			swap(A,i,end);
			end--;
			i--;    //判断交换后改位的数
		}
	}
}
void swap(int[] A,int i,int j){
	int tmp=A[i];
	A[i]=A[j];
	A[j]=tmp;
}

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