leetcode[75] Sort Colors

Given an array with n objects colored red, white or blue, sort them in-place 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.

Example:

Input: [2,0,2,1,1,0]

Output: [0,0,1,1,2,2]

 

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 a one-pass algorithm using only constant space?

 

题目大意：

给了红白蓝三种颜色，分别标识为0,1,2，按红白蓝顺序排序，并将相同颜色放到一起。

 

解法：

其实使用快排就可以了，但是题目的Follow up中说只有三种颜色，那么我们可以记录0,1,2的个数，最后将数组进行覆盖就可以了。

C++：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        vector<int>counts(3,0);
        for(int i=0;i<nums.size();i++){
            counts[nums[i]]++;
        }
        int i=0;
        while(i<counts[0]){
            nums[i]=0;
            i++;
        }
        while(i<counts[0]+counts[1]){
            nums[i]=1;
            i++;
        }
        while(i<nums.size()){
            nums[i]=2;
            i++;
        }
    }
};