367. Valid Perfect Square

367. Valid Perfect Square (Easy)

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

Example 2:

Input: 14
Output: false

Related Topics:
Math, Binary Search

Similar Questions:

• Sqrt(x) (Easy)
• Sum of Square Numbers (Easy)

Solution 1.

// OJ: https://leetcode.com/problems/valid-perfect-square/
// Author: github.com/lzl124631x
// Time: O(sqrt(num))
// Space: O(1)
class Solution {
public:
    bool isPerfectSquare(int num) {
        long i = 0;
        while (i * i < num) ++i;
        return i * i == num;
    }
};

Solution 2. Binary Search

// OJ: https://leetcode.com/problems/valid-perfect-square/
// Author: github.com/lzl124631x
// Time: O(log(num))
// Space: O(1)
class Solution {
public:
    bool isPerfectSquare(int num) {
        long L = 1, R = num;
        while (L <= R) {
            long M = L + (R - L) / 2;
            if (M * M == num) return true;
            if (num / M < M) R = M - 1;
            else L = M + 1;
        }
        return false;
    }
};