367. Valid Perfect Square (Easy)

https://leetcode.com/problems/valid-perfect-square/

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.

Example 1:

Input: 16
Output: true

Input: 14
Output: false

Solutions

class Solution {

    public boolean isPerfectSquare(int num) {
        long x = num;
        while (x * x > num) {
            x = (x + num / x) / 2;
        }

        return x * x == num;
    }
}

Incorrect Solutions

class Solution {

    // Time Limit Exceeded

    public boolean isPerfectSquare(int num) {
        int left = 1;
        int right = num;

        while (left <= right) {
            int mid = (left + right) / 2;
            if (mid * mid == num) {
                return true;
            } else if (mid * mid > num) {
                right = mid - 1;
            } else if (mid * mid < num) {
                left = mid + 1;
            }

            // this is important, otherwise it loop forever
            if (left == right) {
                break;
            }
        }

        return false;
    }
}

References

• 367. Valid Perfect Square 检验完全平方数