David Raleche

What is a palindrome ? Find the definition below from the online oxford

a word, phrase, or sequence that reads the same backward as forward, e.g., madam or nurses run.

In the example below only a word palindrome could be identified. Additional conditions would be necessary to analyze phrase or sequences

function isPalindrome($str): bool
{
    $str_length = strlen($str);
    $mid = round($str_length / 2);
    $last_character = $str_length - 1;

    for ($i = 0; $i <= $mid; $i++) {
        if ($str[$i] != $str[$last_character]) {
            return false;
        }
        $last_character--;
    }
    return true;
}

var_dump( isPalindrome('madam'));
var_dump( isPalindrome('nursesrun'));
var_dump( isPalindrome('davad'));
var_dump( isPalindrome('avaava'));

What is the run time complexity ?

It sounds to be O(n/2)

What is the space complexity ?

We do not allocate memory except for few variables as $i, $mid and $last_character

O(1)