LeetCode 75. Sort Colors （颜色分类）：三路快排

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?

题目要求是，时间复杂度O(n) 空间复杂度O(1) 对于没有听过三路快排的窝来说，这个medium比hard难得多好趴。。。

先说原理，

快速排序就是找一个基准v，通过双向指针，把<v的值放在左边，>=v的值放在右边，然后递归。

三路，就是 <v， =v， >v。那么如何分成三分呢？

设有一个数组 [0...n] 遍历数组，index 表示遍历到的位置。

找两个分界位置，lt 和 rt 用来表示 =v 和 >v 的最小位置。

如果遇到小于 v 的就让其和lt交换，同时将 lt++。等于 v 不需要做操作。大于 v 就将 lt-- 其和 lt 作交换

之后将 <v 和 >v 的两段递归排序就可以啦。

那么这题，只是三路排序的一个思路~~

/*
 * @lc app=leetcode id=75 lang=javascript
 *
 * [75] Sort Colors
 */
/**
 * @param {number[]} nums
 * @return {void} Do not return anything, modify nums in-place instead.
 */
var sortColors = function(nums) {
    let n = nums.length;
    let lt = 0, // =v 的第一个
        rt = n; // >v 的第一个

    let index = 0;
    let v = 1;

    while (index < n && index < rt) {
        if (nums[index] > v) {
            rt--;
            swap(index, rt);
        } else if (nums[index] === v) {
            index++;
        } else if (nums[index] < v) {
            swap(lt, index);
            lt++;
            index++;
        }
    }

    function swap(i, j) {
        let t = nums[i];
        nums[i] = nums[j];
        nums[j] = t;
    }
};

来源：oschina

链接：https://my.oschina.net/u/4276314/blog/3533066

标签

RED

Here

javascript