215. Kth Largest Element in an Array

import random
class Solution(object):
    # @param k & A a integer and an array
    # @return ans a integer
    def findKthLargest(self, A, k):
        return self.quickselect(0, len(A) - 1, A, len(A) - k)
        
    def quickselect(self, start, end, A, k):
        if start == end:    return A[start]
        mid = self.partition(start, end, A)
        if mid == k:    return A[k]
        elif mid > k:   return self.quickselect(start, mid - 1, A, k)
        else:   return self.quickselect(mid + 1, end, A, k)
        
    def partition(self, start, end, A):
        pivotIndex = random.randrange(start, end + 1)
        pivot = A[pivotIndex]
        A[end], A[pivotIndex] = A[pivotIndex], A[end]
        mid = start
        for i in xrange(start, end):
            if A[i] < pivot:
                A[mid], A[i] = A[i], A[mid]
                mid += 1
        A[mid], A[end] = A[end], A[mid]
        return mid