1.问题描述

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

2. 解题代码

2.1. 基本解法

public class Solution {
    public void SortColors(int[] nums) {
        int color_1=0;
        int color_2=0;
        int color_3=0;

        for(int i=0;i<nums.Length;i++)
        {
            if(nums[i]==0)
            {
                color_1++;
            }
            else if(nums[i]==1)
            {
                color_2++;
            }
            else if(nums[i]==2)
            {
                color_3++;
            }
        }
        int startIndex=0;
        for(int i=0;i<color_1;i++)
        {
            nums[startIndex++]=0;
        }
        for(int i=0;i<color_2;i++)
        {
            nums[startIndex++]=1;
        }
        for(int i=0;i<color_3;i++)
        {
            nums[startIndex++]=2;
        }
    }
}

2.2. 单指针解法

public class Solution {
    
    public void SortColors(int[] nums) {
        int n = nums.Length;
        int p0 = 0;
        for (int i = 0; i < n; ++i) {
            if (nums[i] == 0) {
                Swap(nums,i,p0);
                ++p0;
            }
        }
        for (int i = p0; i < n; ++i) {
            if (nums[i] == 1) {
                Swap(nums,i,p0);
                ++p0;
            }
        }
    }

    public void Swap(int[] nums,int indexA,int indexB)
    {        
        int temp = nums[indexA];
        nums[indexA] = nums[indexB];
        nums[indexB] = temp;
    }
}

2.3. 双指针解法

public void SortColors(int[] nums) {
    int n = nums.Length;
    int p0 = 0, p2 = n - 1;
    for (int i = 0; i <= p2; ++i) {
        while (i <= p2 && nums[i] == 2) {
            Swap(nums,i,p2);
            --p2;
        }
        if (nums[i] == 0) {
            Swap(nums,i,p0);
            ++p0;
        }
    }
}

public void Swap(int[] nums,int indexA,int indexB)
{        
    int temp = nums[indexA];
    nums[indexA] = nums[indexB];
    nums[indexB] = temp;
}