PHP 逆轉字串與逆轉句子

 1  <?php 2     #顛倒字串 3      4     #將字串從頭和尾向中間遍曆，交換位置 5     function cstrrev(&$str, $begin, $len) { 6         $i = $begin; 7         $j = $begin + $len - 1; 8         while ($i < $j) { 9             $temp = $str[$i];10             $str[$i] = $str[$j];11             $str[$j] = $temp;12             $i++;13             $j--;14         }15     }16 17     #逆轉句子，但不逆轉單詞18 19     #第一種方法，先將句子整個逆轉，再將單詞逐個逆轉，複雜度為O(2n)20     function sentence_rev(&$s) {21         cstrrev($s, 0, strlen($s));22         $i = 0;23         $j = 0;24         while ($j < strlen($s)) {25             #遇到空格即為一個單詞，將該單詞逆轉26             if ($s[$j] == " " || $j == strlen($s) - 1) {27                 cstrrev($s, $i, $j - $i);28                 $i = $j + 1;29             }30             $j++;31         }32     }33 34     #第二種方法，每次遇到空格即為一個單詞，將該單詞入棧，然後依次出棧，時間複雜度為O(n),空間複雜度為O(n),其中k為單詞數35     function sentence_rev2($s) {36         $stack = array();37         $word = "";38         $i = 0;39         while ($i < strlen($s)) {40             if ($i == strlen($s) - 1) {41                 $word .= $s[$i];42                 array_push($stack, $word);43             }44             if ($s[$i] == " ") {45                 array_push($stack, $word);46                 $word = "";47             } else {48                 $word .= $s[$i];49             }50             $i++;51         }52 53         $sr = "";54         while (!empty($stack)) {55             $sr .= array_pop($stack) . " ";56         }57 58         return substr($sr, 0, strlen($sr) - 1);59     }60 61     $str = "abcdefg";62     cstrrev($str, 0, strlen($str));63     echo $str . "<br>";64     $s = "I am alexis";65     sentence_rev($s);66     echo $s . "<br>";67     $s2 = sentence_rev2("You are not alexis");68     echo $s2;69 ?>

gfedcba
alexis am I
alexis not are You