Leetcode.215 | Kth Largest Element in an Array

Leetcode.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

快排partition的思想

• 时间复杂度：O(logn)
• 空间复杂度：O(1)

import random

class Solution:
    def findKthLargest(self, nums: List[int], k: int) -> int:
        n = len(nums)
        left = 0
        right = n - 1
        target = n - k
        while True:
            po = self._partional(left,right,nums)
            if po == target:
                return nums[po]
            elif po > target:
                right = po - 1
            else:
                left = po + 1
                   
    def _partional(self, start: int, end: int, nums: List[int]) -> int:
        a = random.randint(start,end)
        nums[start],nums[a] = nums[a],nums[start]
        r = nums[start]
        # [right,end] > r
        # [start + 1,left] < r
        # [left+1,i] = r
        right = end + 1
        left = start
        i = start
        while i < right:
            if nums[i] > r:
                right -= 1
                nums[right],nums[i] = nums[i], nums[right]
            elif nums[i] < r:
                left += 1
                nums[left], nums[i] = nums[i], nums[left]
                i += 1
            else:
                i += 1
        nums[left],nums[start] = nums[start],nums[left]
        left -= 1
        return left+1

堆的思想