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.

public class Solution {
public int findKthLargest(int[] nums, int k) {
    if (nums == null || nums.length == 0 || k > nums.length) {
        return 0;
    }
    Arrays.sort(nums);
    return nums[nums.length - k];
    }
}
// From reference
public class Solution {
    public int findKthLargest(int[] nums, int k) {
        return quickSelect(nums, k - 1, 0, nums.length - 1);
    }
    
    private int quickSelect(int[] arr, int k, int left, int right){
        int pivot = arr[(left + right) / 2];
        int orgL = left, orgR = right;
        while(left <= right){
            // 从右向左找到第一个小于枢纽值的数
            while(arr[left] > pivot){
                left ++;
            }
            // 从左向右找到第一个大于枢纽值的数
            while(arr[right] < pivot){
                right --;
            }
            // 将两个数互换
            if(left <= right){
                swap(arr, left, right);
                left ++;
                right --;
            }
        }
        // 最后退出的情况应该是右指针在左指针左边一格
        // 这时如果右指针还大于等于k,说明kth在左半边
        if(orgL < right && k <= right) return quickSelect(arr, k, orgL, right);
        // 这时如果左指针还小于等于k,说明kth在右半边
        if(left < orgR && k >= left) return quickSelect(arr, k, left, orgR);
        return arr[k];
    
    }
    
    private void swap(int[] arr, int idx1, int idx2){
        int tmp = arr[idx1] + arr[idx2];
        arr[idx1] = tmp - arr[idx1];
        arr[idx2] = tmp - arr[idx2];
    
    }
}

reference: very good source
https://segmentfault.com/a/1190000003704825

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