PHP 逆转字符串与逆转句子

 <?php
    #颠倒字符串
    
    #将字符串从头和尾向中间遍历，交换位置
    function cstrrev(&$str, $begin, $len) {
        $i = $begin;
        $j = $begin + $len - 1;
        while ($i < $j) {
            $temp = $str[$i];
            $str[$i] = $str[$j];
            $str[$j] = $temp;
            $i++;
            $j--;
        }
    }

    #逆转句子，但不逆转单词

    #第一种方法，先将句子整个逆转，再将单词逐个逆转，复杂度为O(2n)
    function sentence_rev(&$s) {
        cstrrev($s, 0, strlen($s));
        $i = 0;
        $j = 0;
        while ($j < strlen($s)) {
            #遇到空格即为一个单词，将该单词逆转
            if ($s[$j] == " " || $j == strlen($s) - 1) {
                cstrrev($s, $i, $j - $i);
                $i = $j + 1;
            }
            $j++;
        }
    }

    #第二种方法，每次遇到空格即为一个单词，将该单词入栈，然后依次出栈，时间复杂度为O(n),空间复杂度为O(n),其中k为单词数
    function sentence_rev2($s) {
        $stack = array();
        $word = "";
        $i = 0;
        while ($i < strlen($s)) {
            if ($i == strlen($s) - 1) {
                $word .= $s[$i];
                array_push($stack, $word);
            }
            if ($s[$i] == " ") {
                array_push($stack, $word);
                $word = "";
            } else {
                $word .= $s[$i];
            }
            $i++;
        }

        $sr = "";
        while (!empty($stack)) {
            $sr .= array_pop($stack) . " ";
        }

        return substr($sr, 0, strlen($sr) - 1);
    }

    $str = "abcdefg";
    cstrrev($str, 0, strlen($str));
    echo $str . "<br>";
    $s = "I am alexis";
    sentence_rev($s);
    echo $s . "<br>";
    $s2 = sentence_rev2("You are not alexis");
    echo $s2;
?>

gfedcba
alexis am I
alexis not are You