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.

Example 1:

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

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 in python:

class Solution:
    def findKthLargest(self, nums: List[int], k: int) -> int:
        def quickselect(arr, left, right):
            privot = arr[left]
            i = left
            j = right
            while i != j:
                while i < j and arr[j] >= privot:
                    j -= 1
                arr[i] = arr[j]
                while i < j and arr[i] <= privot:
                    i += 1
                # print("i:",i,"j:",j)
                arr[j] = arr[i]
            arr[i] = privot
            if i < len(arr) - k:
                return quickselect(arr, i+1, right)
            elif i > len(arr) - k:
                return quickselect(arr, left, i)
            else:
                return arr[i]
        return quickselect(nums, 0, len(nums)-1)

Complexity analysis:

• Time complexity: E(O(N))
• Space complexity: O(1)