LeetCode 215. Kth Largest Element in an Array

最新推荐文章于 2023-12-15 19:39:48 发布

原创最新推荐文章于 2023-12-15 19:39:48 发布 · 89 阅读

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博客主要围绕在未排序数组中查找第k大元素展开，指出该元素是排序顺序中的第k大，而非第k个不同元素，还提到此问题和选前k小（或大）元素本质相同。

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Find the kth largest element in an unsorted array. Note that it is the kth largest element in the sorted order, not the kth distinct element.

Example 1:

Input: [3,2,1,5,6,4] and k = 2
Output: 5

Example 2:

Input: [3,2,3,1,2,4,5,5,6] and k = 4
Output: 4

事实上，这道题和选前k小（或大）的元素是一模一样的。

import java.util.*;

class Solution {    
    
    public int findKthLargest(int[] nums, int k) {
        
        shuffle(nums);//乱序
        
        k = nums.length-k; //倒数第k 转为 正数
        int low = 0;
        int high = nums.length-1;
        while(low<high){
            int j = partition(nums,low,high);
            if(j==k){
                break;
            }
            else if(j<k){
                low = j+1;
            }
            else if(j>k){
                high = j-1;
            }
        }
        return nums[k];
    }
    
    private int partition(int[] nums,int low,int high){
        int i = low;
        int j= high+1;
        while(true){
            while(nums[++i]<nums[low] ){
                if(i==high) break;
            }
            while( nums[low]<nums[--j] ){
                // if(j==low) break;
            }
            if(i>=j){
                break;
            }
            swap(nums,i,j);
        }
        swap(nums,low,j);
        return j;
    } 
    
    private void swap(int[] nums,int i ,int j){
        int t = nums[i];
        nums[i] = nums[j];
        nums[j] = t;
    }
    
    private void shuffle(int nums[]) {
        Random random = new Random();
        for(int ind = 1; ind < nums.length; ind++) {
            int r = random.nextInt(ind + 1);
            swap(nums, ind, r);
        }
    }
}