[leetcode] 【排序】 75. Sort Colors

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.

click to show follow up.

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?

题意

数组仅包含红白蓝三种元素，用0、1、2表示。对数组进行排序，最好不使用库的排序。

使用计数的方式，需要遍历两遍，要求你用一遍遍历完成。空间复杂度为常数。

题解

要一遍完成，则设置两个下标，红的和蓝的，红的往后移动，蓝的往前移动。

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