LeetCode Top 100 Liked Questions 75. Sort Colors (Java版； Medium)

Posted 2023-01-18

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LeetCode Top 100 Liked Questions 75. Sort Colors (Java版; Medium)

题目描述

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.

Note: You are not suppose to use the librarys sort function for this problem.

Example:

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

class Solution 
    public void sortColors(int[] nums) 
        int n = nums.length;
        if(n<=1)
            return;
        
        partition(nums, 0, n-1);
    

    private void partition(int[] nums, int L, int R)
        int small=L-1, big=R+1;
        int p = L;
        while(p<big)
            if(nums[p] < 1)
                swap(nums, ++small, p++);
            else if(nums[p] > 1)
                swap(nums, --big, p);
            else
                p++;
            
        
    

    private void swap(int[] arr, int i, int j)
        int tmp = arr[i];
        arr[i] = arr[j];
        arr[j] = tmp;
    

荷兰国旗, 实际上就是执行了一次partition; 关键点: 循环终止条件

第二次做; 最朴素的想法: 桶排序; 时间复杂度O(N^2), 不推荐这个方法

class Solution 
    public void sortColors(int[] nums) 
        //用计数排序解决一次
        int[] arr = new int[3];
        for(int a : nums)
            arr[a]++;
        //从桶中倒出来
        int j = 0;
        for(int i=0; i<arr.length; i++)
            while(arr[i]>0)
                nums[j++] = i;
                arr[i]--;
            
        
    

第一次做, 荷兰国旗问题, 遍历一次数组即可; partition将数组分成三部分:小于区,等于区,大于区; 口诀:换扩跳, 换扩, 跳; 细节: 必须用1作为划分值, 因为题目要求1在等于区; 小于区向右扩, 大于区向左扩, 剩下的是等于区;

/*
荷兰国旗问题,就是一轮partition过程
将数组分成三个部分
细节:划分值必须选1, 因为题目规定了1在中间, 所以不能选0或者2在中间
*/
class Solution 
    public void sortColors(int[] nums) 
        if(nums==null || nums.length==0)
            return;
        int small = -1;//小于区向右扩
        int big = nums.length; //大于区向左扩
        int pivot = 1; //划分值必须选1, 因为题目规定了1在中间
        int i = 0;
        while(i<big)
            if(nums[i] < pivot)
                swap(nums, i++, ++small);
            else if(nums[i] > pivot)
                swap(nums, i, --big);
            else
                i++;
        
    
    public static void swap(int[] arr, int i, int j)
        int temp = arr[i];
        arr[i] = arr[j];
        arr[j] = temp;