75. 颜色分类

目录

• 题目描述
• 思路
• 代码实现

题目描述

Given an array with n objects colored red(红), white(白) or blue(蓝),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: //计数排序
A rather straight forward solution is a two-pass algorithm(两趟算法) 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?

思路

基于partation思想,小于1的数放在左边，大于1的数放在右边，等于1的数不动

代码实现

class Solution {
public:
    void sortColors(vector<int>& nums) {
        
        int n = nums.size();
        int zero = -1;//已排好的边界
        int two = n;//尾指针对运用，这个问题使用了这两个尾指针
        //i,当前要判断的元素下标，从左往右扫描，当遇到右边的尾指针时终止操作
        //所以这里使用了while
        int i = 0;
        while(i<two)
        {
            if(nums[i] == 0)
            {
                swap(nums[++zero],nums[i]);
                i++;
            }
            else if(nums[i] == 2)
            {
                swap(nums[i],nums[--two]);
            }
            else 
                i++;
        }
        
    }
};