Leetcode 75. Sort Colors

题目

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

**Level: ** Medium

Discription:
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.

Example 1:

Input: [2,0,2,1,1,0]
Output: [0,0,1,1,2,2]

Note:

• You are not suppose to use the library's sort function for this problem.
• Could you come up with a one-pass algorithm using only constant space?

代码

class Solution {
public:
    void sortColors(vector<int>& nums) {
        if(nums.size()<=1)
            return;
        int fa=0;
        int fc=nums.size()-1;
        int fb=0;
        while(fb<=fc)
        {
            if(nums[fb]==0)
            {
                swap(nums[fa], nums[fb]);
                fa++;
                fb++;
            }
            else if(nums[fb]==2)
            {
                swap(nums[fb], nums[fc]);
                fc--;
            }
            else
            {
                fb++;
            }
        }
        return;  
    }
};

思考

• 算法时间复杂度为O((n))，空间复杂度为O((1) )。
• 题目要求一次遍历，in-place完成，采用三指针，首尾两个指针，中间一个指针。中间指针取到1时，往后移动，如果为0，则和首指针互换，首指针和中间指针均往后移一位。如果为2，则和尾指针互换，尾指针向前移一位。当中间指针移动到比尾指针还大时跳出循环。