215. Kth Largest Element in an Array

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.

思路：借用快排中的partition的思想

代码如下(已通过leetcode)

public class Solution {

public int findKthLargest(int[] nums, int k) {

int position=findKthLargest(nums, 0, nums.length-1, k);

return nums[position];

}

public int findKthLargest(int[] nums,int start,int end, int k) {

int n = nums.length;

int position=partition(nums, start,end);

if(position==n-k) return position;

else{

if(position<n-k) return findKthLargest(nums, position+1,end,k);

else return findKthLargest(nums, start, position-1, k);

}

}

private int partition(int[] nums, int low, int high) {

// TODO Auto-generated method stub

int i=low,j=high;

int key=nums[low];

while(i<j) {

while(nums[j]>=key&&j>i) {

j--;

}

int temp=nums[i];

nums[i]=nums[j];

nums[j]=temp;

while(nums[i]<=key&&i<j) {

i++;

}

temp=nums[i];

nums[i]=nums[j];

nums[j]=temp;

}

return i;

}

}