PHP string sorting in reverse order [strrev function, dichotomy, loop method, recursive method]

The example in this article summarizes the implementation method of reverse order arrangement of PHP strings. Share it with everyone for your reference, the details are as follows:

Regarding the reverse order of strings, the simplest test code using the PHP function strrev() is as follows:

header('Content-type: text/html; charset=utf-8');
$str = implode('', range(9, 0));
print &#39;< p><strong>Before reversed: </strong>&#39;.$str.&#39;< /p>&#39;;
print &#39;< p>< strong>After reversed: < /strong>&#39;.strrev($str).&#39;< /p>&#39;;
/*
输出如下：
Before reversed: 9876543210
After reversed: 0123456789
*/

How to implement this without using the built-in PHP function strrev()? Three additional methods (bisection method, loop method, recursive method) are given here, but no performance test is performed.

(PS: In order to make it easier for everyone to read the code, the online php code formatting tool http://tools.jb51.net/code/jb51_php_format of this site is used for formatting.)

1. Dichotomy method

/**
* 二分法实现字符串逆序排列
* @param string $str 源字符串
* @return string 返回逆序后的字符串
*/
function reverse($str='') {
  $len = strlen($str);
  //不能使用count或sizeof
  $mid = floor($len/2);
  for ($i=0; $i<$mid; $i++) {
    $temp = $str[$i];
    $str[$i] = $str[$len-$i-1];
    $str[$len-$i-1] = $temp;
  }
  return $str;
}

2. Circular method

/**
* 循环实现对字符串的逆序排列(效率比二分法低)
* @param string $str 源字符串
* @return string 返回逆序后的字符串
*/
function reverse($str='') {
  $result = '';
  for ($i=1; $i<=strlen($str); $i++) {
    $result .= substr($str, -$i, 1);
  }
  return $result;
}

##3. Recursive method

/**
* 递归实现对字符串的逆序排列(效率低)
* @param string $str 源字符串
* @return string 返回逆序后的字符串
*/
function reverse($str='') {
  static $result = '';
  /* 用堆栈来理解递归调用 */
  if (strlen($str) > 0) {
    reverse(substr($str, 1));
    $result .= substr($str, 0, 1);
    //此句必须放在上一语句之后
  }
  return $result;
}

I hope this article will be helpful to everyone in PHP programming.

For more PHP string arrangement in reverse order [strrev function, dichotomy, loop method, recursive method] related articles, please pay attention to the PHP Chinese website!