Problem description:

Given a positive integer num, write a function which returns True if num is a perfect square else False.

Follow up: Do not use any built-in library function such as sqrt.

Example 1:

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Input: num = 16
Output: true

Example 2:

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Input: num = 14
Output: false

Constraints:

  • 1 <= num <= 2^31 - 1

Solution:

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class Solution:
    def isPerfectSquare(self, num: int) -> bool:
        l, r = 0, num
        while l <= r:
            mid = l + (r-l) //2
            if mid **2 == num:
                return True
            elif mid **2 > num:
                r = mid-1
            else:
                l = mid+1
        
        return False

time complexity: $O()$
space complexity: $O()$
reference:
related problem: