[Leetcode] Sort Colors

描述

Given an array with n objects colored red, white or blue, sort them 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.

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 an one-pass algorithm using only constant space?

分析

在一个数组中只有 0,1,2 这三种元素若干个，将数组排序，要求一次遍历完成。

一次遍历的问题通常可以考虑双指针的方法，考虑使用两个指针，分别指向元素“1”的首末，则每当有新元素时，只需将它插入到其中一个指针的位置并且更新指针就可以了。

代码

class Solution {
public:
void sortColors(vector<int>& nums) {
int s1 = 0, s2 = 0;
for (int i = 0; i < nums.size(); i++) {
if (nums[i] == 0) {
nums[i] = nums[s2]; nums[s2] = nums[s1]; nums[s1] = 0; s1++; s2++;
} else if (nums[i] == 1) {
nums[i] = nums[s2]; nums[s2] = 1; s2++;
}
}
}
};