Leetcode 75. Sort Colors 色彩排序 解题报告

1 解题思想

这道题是说，现在有红色，白色，蓝色三种，分别用0，1，2表示，现在有一个表示这样的数组，然后请把它进行排序，使得相同颜色的在一起，并且整体按照0 1 2排序。

这道题其实用不到排序，首先我们扫描一下有几个0，几个1，几个2就可以了，然后输出的时候，按照0，1，2统计的个数，顺序的赋值给数组就可以了。

2 原题

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?

3 AC解

public class Solution {
    /** * 思路，直接统计颜色，然后从小大大的输出就好了 * colorcount代表当前rgb每个颜色各出现多少次 * 统计完后，直接覆盖nums就可以 * */
    public void sortColors(int[] nums) {
        int colorcount[]=new int[]{0,0,0};
        int i,j;
        for(i=0;i<nums.length;i++) colorcount[nums[i]]++;
        i=0;
        for(j=0;j<3;j++){
            while(colorcount[j]-->0){
                nums[i++]=j;
            }
        }
    }
}