215. Kth Largest Element in an Array

Description

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.

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

Solution

MinHeap, O(n * logk), S(k)

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

Quick-sort partition, time O(n), space O(1)

class Solution {
    public int findKthLargest(int[] nums, int k) {
        return findKth(nums, 0, nums.length - 1, nums.length - k);
    }
    // find the kth (start from 0) element in sorted nums
    private int findKth(int[] nums, int start, int end, int k) {
        int pivotIndex = partition(nums, start, end);
        
        if (pivotIndex < k) {
            return findKth(nums, pivotIndex + 1, end, k);
        } else if (pivotIndex > k) {
            return findKth(nums, start, pivotIndex - 1, k);
        } else {
            return nums[k];
        }
    }
    
    private int partition(int[] nums, int start, int end) {
        int pivot = nums[end];
        for (int i = start; i < end; ++i) {
            if (nums[i] <= pivot) {
                swap(nums, start++, i);
            }
        }
        
        swap(nums, start, end);
        return start;
    }
    
    private void swap(int[] nums, int i, int j) {
        int tmp = nums[i];
        nums[i] = nums[j];
        nums[j] = tmp;
    }
}