python leetcode判断一个数是否为完全平方数

class Solution:
    def isPerfectSquare(self, num):
        """
        :type num: int
        :rtype: bool
        """
        start = 1
        end = num
        while start <= end:
            mid = start +(end-start) //2
            sqr = mid ** 2
            if sqr < num:
                start =mid+1
            elif sqr > num:
                end = mid -1
            else:
                return True
        return False