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• 原题连接：215. 数组中的第K个最大元素

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1- 思路

快速选择

• 第 k 大的元素的数组下标： int target = nums.length - k

1- 根据 partition 分割的区间来判断当前处理方式

• 如果返回的 int 等于 target 说明找到了，直接返回
• 如果返回的 int 小于 target 说明要在当前区间的右侧寻找，也就是 [pivotIndex+1,right]
• 如果返回的 int 大于 target 说明要在当前区间的左侧寻找，也就是 [left,pivotIndex-1]

2- 实现 partition 随机选取一个 pivotIndex 分割区间

• 2-1 随机选择一个下标
• 2-2 交换 left 和 随机下标
• 2-3 将随机下标的元素值设置为 pivot
• 2-4 定义 le、ge 下标 使用 while(true)
  • 使得 le 指向的元素始终小于 pivot
  • 使得 ge 指向的元素始终大于 pivot

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2- 实现

⭐215. 数组中的第K个最大元素——题解思路

import java.util.Random;
class Solution {static Random random = new Random(System.currentTimeMillis());public int findKthLargest(int[] nums,int k){return quickSelect(nums,0,nums.length-1,nums.length-k);}public int quickSelect(int[] nums,int left,int right,int kIndex){if(right==left){return nums[left];}//int pivotIndex = partition(nums,left,right);if(pivotIndex == kIndex){return nums[kIndex];}else if( pivotIndex>kIndex){return quickSelect(nums,left,pivotIndex-1,kIndex);}else{return quickSelect(nums,pivotIndex+1,right,kIndex);}}public int partition(int[] nums,int left,int right){int randomIndex = left + random.nextInt(right-left+1);swap(nums,left,randomIndex);int mid = nums[left];int le = left+1;int ge = right;while(true){while(le<=ge && nums[le] < mid){le++;}while(le<=ge && nums[ge] > mid){ge--;}if(le>=ge){break;}swap(nums,le,ge);le++;ge--;}swap(nums,left,ge);return ge;}public void swap(int[] nums,int left,int right){int tmp = nums[left];nums[left] = nums[right];nums[right] = tmp;}}

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3- ACM实现

public class kthNums {static Random random = new Random(System.currentTimeMillis());public static int findK(int[] nums,int k){// 快速选择 ，传四个参数return quickSelect(nums,0,nums.length-1,nums.length-k);}public static int quickSelect(int[] nums,int left,int right,int kIndex){if(right==left){return nums[left];}//int pivotIndex = partition(nums,left,right);if(pivotIndex == kIndex){return nums[kIndex];}else if( pivotIndex>kIndex){return quickSelect(nums,left,pivotIndex-1,kIndex);}else{return quickSelect(nums,pivotIndex+1,right,kIndex);}}public static int partition(int[] nums,int left,int right){int randomIndex = left + random.nextInt(right-left+1);swap(nums,left,randomIndex);int mid = nums[left];int le = left+1;int ge = right;while(true){while(le<=ge && nums[le] < mid){le++;}while(le<=ge && nums[ge] > mid){ge--;}if(le>=ge){break;}swap(nums,le,ge);le++;ge--;}swap(nums,left,ge);return ge;}public static void swap(int[] nums,int left,int right){int tmp = nums[left];nums[left] = nums[right];nums[right] = tmp;}public static void main(String[] args) {Scanner sc = new Scanner(System.in);String input = sc.nextLine();String[] parts = input.split(" ");int[] nums = new int[parts.length];for(int i = 0 ; i < nums.length ; i++){nums[i] = Integer.parseInt(parts[i]);}System.out.println("输入K");int k = sc.nextInt();System.out.println("结果是"+findK(nums,k));}
}