Leetcode 215. Kth Largest Element in an Array 数组中第K大的数 in Java

215. Kth Largest Element in an Array

• Total Accepted: 76233
• Total Submissions: 213997
• Difficulty: Medium

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.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

Note: 
You may assume k is always valid, 1 ≤ k ≤ array's length.

public class Solution {
    public int findKthLargest(int[] nums, int k) {
        return nums[partition(nums,0,nums.length-1,k)];
    }
    
    public int partition(int[] nums,int left,int right,int k){  //类似quicksort
        if(left==right) return left;
        
        int standard = nums[left];  
        int m = left;  //m代表比standard小的最后一个数的index
        int n = left+1;  //n代表当前用于比较数的index
          
        while(n<=right){  
            if(nums[n]<standard) {  
                swap(nums, ++m, n);
//++m后代表就是大于standard的第一个数字的index，然后与n处的数交换，m又成为了比standard小的最后一个数的index
            }  
            n++;  
        }
        swap(nums,left, m);//最后把第一个standard和最后一个小于他的数交换，那个小的数到了第一，standard到了中间
        
        //至此完成了以standard为标准的划分
        
        int mid=m-left;         //mid和lenTmp代表的是以left-right窃取的这段数组对应的值
        int lenTmp=right-left+1;//m代表的是整个nums数组对应的值
        
        if(lenTmp-mid==k)     return m;  //注意何处用mid和lenTmp,何处用m
        else if(lenTmp-mid<k) return partition(nums,left,m-1,k-(lenTmp-mid));
        else                  return partition(nums,m+1,right,k);
    }
    
    public void swap(int[] nums, int i, int j) {  
        int temp = nums[i];  
        nums[i] = nums[j];  
        nums[j] = temp;  
    }  
}