215. Kth Largest Element in an Array

Given an integer array nums and an integer k, return the kth largest element in the array.

Note that it is the kth largest element in the sorted order, not the kth distinct element.

You must solve it in O(n) time complexity.

 

Example 1:

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

Example 2:

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

 

Constraints:

• 1 <= k <= nums.length <= 105
• -104 <= nums[i] <= 104

---

#https://leetcode.com/problems/kth-largest-element-in-an-array/description/

'''
1. 아이디어 :
    1)  heapq를 사용할 수 있다
    2)  배열을 정렬할 수 있다.
2. 시간복잡도 :
    1)  O(n) + O(k log n) = O(n log k)
    2)  O(n) = O(nlogn)
    * 이론상 최악의 경우, 1)번이 더 빠르지만, 실제로는 2)번이 더 빠르다
3. 자료구조 :
    1)  heapq
    2)  배열
'''

#1)
class Solution:
    def findKthLargest(self, nums: List[int], k: int) -> int:
        ans = []
        for num in nums:
            ans.append([-num,num])
        heapq.heapify(ans)
        for i in range(k-1):
            heapq.heappop(ans)
        return ans[0][1]

#2)
# class Solution:
#     def findKthLargest(self, nums: List[int], k: int) -> int:
#         return sorted(nums, reverse=True)[k-1]