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;
}
};