Leetcode-215题:Kth Largest Element in an Array

题目

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.

For example,
Given [3,2,1,5,6,4] and k = 2, return 5.

思路

利用快速排序的思想，用数组头将数组两分，左边比它小，右边比它大。如果这个元素正好是第k个，直接返回；如果比k大，则从比它小的那些数中找出第k大的；否则，从比它大的数中找出相应的数字。

代码

class Solution(object):

    def quick_sort(self, nums, k):
        i = 0
        for j in range(1,len(nums)):
            if nums[j] > nums[0]:
                i += 1
                nums[i],nums[j] = nums[j],nums[i]
        nums[i],nums[0] = nums[0],nums[i]
        if i == k-1:
            return nums[i]
        elif i > k-1:
            return self.quick_sort(nums[:i], k)
        else:
            return self.quick_sort(nums[i+1:], k-i-1)

    def findKthLargest(self, nums, k):
        """
        :type nums: List[int]
        :type k: int
        :rtype: int
        """
        if nums==None or len(nums)==0 or k<1:
            return -1
        return self.quick_sort(nums, k)