1. 颜色分类

. - 力扣（LeetCode）. - 备战技术面试？力扣提供海量技术面试资源，帮助你高效提升编程技能,轻松拿下世界 IT 名企 Dream Offer。https://leetcode.cn/problems/sort-colors/description/

代码

class Solution {
    public void sortColors(int[] nums) {
        int left = -1 ,  right = nums.length;
        int i = 0;
        while( i < right){
            if(nums[i] == 0){
                swap(nums,++left , i++);
            }else if( nums[i] == 1){
                i++;
            }else{
                swap(nums,--right ,i);
            }
        }
    }
    private static void swap(int[] arr , int left , int right){
        int tmp = arr[left];
        arr[left] = arr[right];
        arr[right] = tmp;
    }
}

2. 快排

. - 力扣（LeetCode）

代码

class Solution {
    public int[] sortArray(int[] nums) {
        qsort(nums,0 , nums.length -1);
        return nums;
    }
    private static void qsort(int[] arr, int l , int r){
        if(l >= r) return ;
        int left = l - 1 , right = r + 1;
        //随机选择一个基准元素
        int povit = arr[new Random().nextInt( r - l + 1) + l];
        int i = l;
        //按照基准元素将数组划分成3块
        while( i < right){
            if(arr[i] < povit) swap(arr , ++left , i++);
            else if(arr[i] == povit) i++;
            else swap(arr , --right , i);
        }
        //继续划分
        qsort(arr, l , left);
        qsort(arr , right , r);
    }

    private static void swap(int[] arr , int a , int b){
        int tmp = arr[a];
        arr[a] = arr[b];
        arr[b] = tmp;
    }
}

3. 第 K 大

代码

class Solution {
    public int findKthLargest(int[] nums, int k) {
        return quickselect(nums, 0, nums.length - 1, k);
    }

    public static int quickselect(int[] nums, int l, int r, int k) {
        if( l == r) return nums[l];
        int povit = nums[new Random().nextInt(r - l + 1) + l];
        int left = l - 1 , right = r + 1 , i = l;
        int a = 0 , b = 0, c = 0;
        while(i < right){
            if(nums[i] < povit) {
                swap(nums , ++left , i++);
                a++;
            }else if(nums[i] == povit){
                i++;
                b++;
            }else{
                swap(nums , --right , i);
                c++;
            }
        }
        //划分后根据数量决定继续去哪里找TOP K
        if(c >= k) return quickselect(nums , right , r, k) ;
        else if(b+c >= k) return povit;
        else return quickselect(nums, l , left, k - b -c) ;
    }
    public static  void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}

4. 返回最小的 K 个数

. - 力扣（LeetCode）

class Solution {
    public int[] inventoryManagement(int[] nums, int k) {
        qsort(nums, 0 , nums.length - 1, k);
        int[] ret = new int[k];
        for(int i = 0 ; i < k; i++){
            ret[i] = nums[i];
        }
        return ret;
    }

    private static void qsort(int[] nums, int l, int r, int k){
        while(l >= r) return;
        int povit = nums[new Random().nextInt(r - l + 1) + l];
        int left = l - 1, right = r + 1, i = l;
        int a , b ,c ; a = 0; b= 0; c = 0;
        while(i < right){
            
            if(nums[i] < povit) {swap(nums, ++left , i++); a++;}
            else if(nums[i] == povit){ i++; b++;}
            else{ swap(nums, --right , i); c++;}
        }
        //划分完毕 根据情况决定去哪一部分继续寻找
        if(a >= k) qsort(nums, l, left, k);
        else if(a + b >= k) return;
        else qsort(nums, right, r, k - a - b);
    }

    private static void swap(int[] nums, int a, int b){
        int tmp = nums[a];
        nums[a] = nums[b];
        nums[b] = tmp;
    }
}