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PHP string reverse order implementation method

墨辰丷Original: 2018-05-26 15:31:403087browse

This article mainly introduces the implementation method of string reverse order in PHP. It summarizes and analyzes the strrev function, bisection method, loop method, recursion method and other commonly used string reverse order operation implementation techniques in the form of examples. Friends who need it can refer to it. The following

is as follows:

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

header(&#39;Content-type: text/html; charset=utf-8&#39;);
$str = implode(&#39;&#39;, range(9, 0));
print &#39;< p><strong>Before reversed: </strong>&#39;.$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 it if the built-in PHP function strrev() is not used? Three additional methods (bisection method, loop method, recursive method) are given here, but no performance test is performed.

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=&#39;&#39;) {
  $len = strlen($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=&#39;&#39;) {
  $result = &#39;&#39;;
  for ($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=&#39;&#39;) {
  static $result = &#39;&#39;;
  /* 用堆栈来理解递归调用 */
  if (strlen($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;
}

The above is the entire content of this article, I hope it will be helpful to everyone's study.

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