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:

Solution

快排partition的思想

• 時間複雜度：O(logn)
• 空間複雜度：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