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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.

解题思路：

（1）堆排序

class Solution:
    def findKthLargest(self, nums: List[int], k: int) -> int:
        """
        :param nums: List[int] 待排序列表
        :param k: int 第k大数值
        :return: int 返回值
        """
        length = len(nums)
        # 构建大顶堆
        for i in range(length // 2 - 1, -1, -1):
            self.adjustHeap(nums, i, length)

        count = 0
        for j in range(length - 1, -1, -1):
            count += 1
            if count == k:
                return nums[0]
            self.swap(nums, 0, j)
            length -= 1
            self.adjustHeap(nums, 0, length)

    def adjustHeap(self, nums, i, length):
        """
        :param nums: 待排序数组
        :param i: 指定以i为根结点排序
        :param length: 待排序数组长度
        :return:
        """
        while True:
            leftChild = i * 2 + 1
            if leftChild >= length:
                return
            if leftChild + 1 < length and nums[leftChild + 1] > nums[leftChild]:
                leftChild += 1
            if nums[leftChild] > nums[i]:
                self.swap(nums, i, leftChild)
                i = leftChild
            else:
                return

    def swap(self, nums, i, j):
        temp = nums[i]
        nums[i] = nums[j]
        nums[j] = temp