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]

算法书中,专门的一节,  用到了和quicksort同样的partition, 算法复杂度是O(n). 直接sort是O(n*log(n))

[CODE]

public class Solution {
    //test case [2, 1], 1
    public int findKthLargest(int[] nums, int k) {
        return findK(nums, nums.length-k, 0, nums.length-1);
    }
    
    private int findK(int[] nums, int k, int i, int j) {
        if(i>=j) return nums[i];
        int m = partition(nums, i, j);
        if(m==k) return nums[m];
        else if(m<k) {
            return findK(nums, k, m+1, j);
        } else {
            return findK(nums, k, i, m-1);
        }
    }
    
    private int partition(int[] nums, int i, int j) {
        int x = nums[i];
        int m = i;
        int n = i+1;
        
        while(n<=j){
            if(nums[n]<x) {
                swap(nums, ++m, n);
            }
            ++n;
        }
        swap(nums,i, m);
        return m;
    }
    
    private void swap(int[] nums, int i, int j) {
        int temp = nums[i];
        nums[i] = nums[j];
        nums[j] = temp;
    }
}