【特别重要!!!快排的partition思想】【leetcode题解】【M】【71】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.

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

Note: 
You may assume k is always valid, 1 ≤ k ≤ array's length.

Credits:
Special thanks to @mithmatt for adding this problem and creating all test cases.

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思路：使用快拍的partition函数：

• O(N) best case / O(N^2) worst case running time + O(1) memory

The smart approach for this problem is to use the selection algorithm (based on the partion method - the same one as used in quicksort).

快速排序算法里的partition函数

快速排序算法里的partition函数用来解决这样一个问题：给定一个数组arr[]和数组中任意一个元素a，重排数组使得a左边都小于它，右边都不小于它。

start、end分别为数组第一个元素和最后一个元素的索引

 // povitIndex为数组中任意选中的数的索引

class Solution(object):

    def partition(self,nums,start,end):
        pivot = nums[end]

        temp = start

        for i in range(start,end):
            if nums[i] < pivot:
                #print nums[i]
                nums[i],nums[temp] = nums[temp],nums[i]
                temp += 1
                #print nums,temp
        nums[end],nums[temp] = nums[temp],nums[end]
        #print nums
        return temp

    def findKthLargest(self, nums, k):
        low = 0
        high = len(nums) - 1

        k = len(nums) - k
        #print k
        while low < high:
            j = self.partition(nums,low,high)
            #print j
            if j < k:
                low = j + 1
            elif j > k:
                high = j - 1
            else:
                break
        return nums[k]

        '''
        nums.sort()
        return nums[-k]
        '''
        """
        :type nums: List[int]
        :type k: int
        :rtype: int
        """