75. Sort Colors**-优快云博客

本文详细解析了LeetCode上75题Sort Colors的解决方案，介绍了如何使用三路快速排序算法来对红、白、蓝三种颜色进行就地排序，避免使用额外的空间。文章通过实例展示了算法的具体实现过程，并探讨了一次遍历的优化方案。

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75. Sort Colors**

https://leetcode.com/problems/sort-colors/

题目描述

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?

解题思路

采用 3 路快排的思路.

C++ 实现 1

lass Solution {
public:
    void sortColors(vector<int>& nums) {

        int lt = -1, gt = nums.size(), i = 0;
        // nums[0...lt] 保存 0
        // nums[lt+1...gt) 保存 1
        // nums[gt...end] 保存 2
        while (i < gt) {
            if (nums[i] == 1)
                i ++;
            else if (nums[i] == 0)
                std::swap(nums[++lt], nums[i++]);
            else
                std::swap(nums[--gt], nums[i]);
        }
    }
};