1 <?php 2 #颠倒字符串 3 4 #将字符串从头和尾向中间遍历，交换位置 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 $j--; 14 } 15 } 16 17 #逆转句子，但不逆转单词 18 19 #第一种方法，先将句子整个逆转，再将单词逐个逆转，复杂度为O(2n) 20 function sentence_rev(&$s) { 21 cstrrev($s, 0, strlen($s)); 22 $i = 0; 23 $j = 0; 24 while ($j < strlen($s)) { 25 #遇到空格即为一个单词，将该单词逆转 26 if ($s[$j] == " " || $j == strlen($s) - 1) { 27 cstrrev($s, $i, $j - $i); 28 $i = $j + 1; 29 } 30 $j++; 31 } 32 } 33 34 #第二种方法，每次遇到空格即为一个单词，将该单词入栈，然后依次出栈，时间复杂度为O(n),空间复杂度为O(n),其中k为单词数 35 function sentence_rev2($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
