LeetCode | 75. Sort Colors

 

题目：

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?

 

代码：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        vector<int> record(3, 0);
        for(int i = 0; i<nums.size(); i++)
            record[nums[i]]++;
        int idx = 0;
        for(int i = 0; i<3; i++)
        {
            for(int j = 0; j<record[i]; j++)
                nums[idx++] = i;
        }
        return;
    }
};