题目
Given a positive integer num, write a function which returns True if num is a perfect square else False.
Note: Do not use any built-in library function such as sqrt.
解题思路
二分搜索n，使得num = n * n
代码
class Solution:
def isPerfectSquare(self, num: int) -> bool:
l, r = 0, num
while l <= r:
m = (l + r) // 2
t = m * m
if t == num:
return True
if t < num:
l = m + 1
else:
r = m - 1
return False
