边工作边刷题：70天一遍leetcode: day 42

Valid Perfect Square

要点：binary search的题，可以扩展到double，就变成<=的情况了
错误点：

• 2以上的情况n/2，但是<2就可能n/2+1。所以high是n/2+1

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