【leetcode】 75. Sort Colors-优快云博客

本文介绍了一个经典的三色排序问题解决方案，即荷兰国旗问题。通过一个一维数组来存放三种颜色的物体，要求相同颜色的物体相邻，并且按照红、白、蓝的顺序排列。文章详细解释了一种使用常数空间的一次遍历算法，该算法不需要额外的排序库函数辅助。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

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?

题解：
下标遍历数组，把红的放在左边，蓝的放在右边。中间保留白色
交换完蓝色之后要保持下标不变，因为交换过来的可能是红色下一次需要放在左边
代码：

class Solution {
public:
    void sortColors(vector<int>& nums) {
        int n = nums.size();
        int blue = n-1;
        int red = 0;
        for(int i = 0;i <= blue; i++){
            if(nums[i] == 2){
                swap(nums[i], nums[blue]);
                blue--;
                i--;
            }else if(nums[i] == 0){
                swap(nums[i], nums[red]);
                red++;
            }
        }
    }
};