[leetcode 367] PHP judges valid perfect squares

Link to the original text: he Xiaodong’s blog

Efficient perfect square number

Give a positive integernumWrite a function ifnumIs a complete squareTrue, otherwise returnFalse。

Note: do not use any built-in library functions, such assqrt。

Example 1:

Input: 16
Output: true

Example 2:

Input: 14
Output: false

Source: leetcode
Links: valid perfect squares

Solution 1

PHP cannot be usedpowFunction, the operation is** 0.5In this way, self multiplying 0.5 times fromPhp5.6.0 startThe effect is the same as root.

code

class Solution {

    /**
     * @param Integer $num
     * @return Boolean
     */
    function isPerfectSquare($num) {
        return $num**0.5 == (int)($num**0.5);
    }
}

Solution 2

By using the properties of complete square numbers,A perfect square is the sum of a series of odd numbersFor example:

1 = 1
4 = 1 + 3
9 = 1 + 3 + 5
16 = 1 + 3 + 5 + 7
25 = 1 + 3 + 5 + 7 + 9
36 = 1 + 3 + 5 + 7 + 9 + 11
....
1+3+...+(2n-1) = (2n-1 + 1) n/2 = n* n

The time complexity is O (sqrt (n)).

code

class Solution {

    /**
     * @param Integer $num
     * @return Boolean
     */
    function isPerfectSquare($num) {
        $start = 1;
        while($num > 0)
        {
            $num - = $start; // decrements to 0 
            $start + = 2; // each time + 2 remains a continuous odd number
        }

        return $num == 0;
    }
}

Solution 3

Binary search

code

class Solution {

    /**
     * @param Integer $num
     * @return Boolean
     */
    function isPerfectSquare($num) {
        $left = 0;
        $right = $num;
        while($left < $right)
        {
            $mid = $right - floor(($right-$left)/2);
            if ($mid * $mid == $num) {
                return true;
            } elseif ($mid * $mid > $num) {
                $right = $mid - 1;
            } else {
                $left = $mid + 1;
            }
        }
        return $left * $left == $num;
    }
}

Reference link: Square

Pass by recommendation algorithm