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.

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

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

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


Solution:

class Solution {
    public int findKthLargest(int[] nums, int k) {
        PriorityQueue<Integer> pq = new PriorityQueue<>();
        for (int val : nums) {
            if (pq.size() < k) {
                pq.offer(val);
            } else if (val > pq.peek()) {
                pq.poll();
                pq.offer(val);
            }
        }
        int result = pq.poll();
        return result;
    }
}

class Solution {
    public int findKthLargest(int[] nums, int k) {
        return quickSelect(nums, 0, nums.length - 1, k);
    }
    
    private int quickSelect(int[] nums, int left, int right, int k) {
        int pivot = left;
        for (int i = left; i < right; i ++) {
            if (nums[i] <= nums[right]) {
                swap(nums, pivot ++, i);
            }
        }
        swap(nums, pivot, right);
        int count = right - pivot + 1;
        if (count == k) {
            return nums[pivot];
        } else if (count < k) {
            return quickSelect(nums, left, pivot - 1, k - count);
        } else {
            return quickSelect(nums, pivot + 1, right, k);
        }
    }
    
    private void swap(int[] nums, int x, int y) {
        int temp = nums[x];
        nums[x] = nums[y];
        nums[y] = temp;
    }
}