PHP full permutation recursive algorithm sample code

Recursionis an importantprogrammingtechnique. This method is used to let a function call itself from within. One example is calculating factorials. The factorial of 0 is specifically defined as 1. The factorial of a larger number is found by calculating 1 * 2 * ..., increasing by 1 each time, until the number whose factorial is to be calculated is reached.

Algorithm Principle
If P represents the full arrangement of n elements, and Pi represents the full arrangement of n elements that does not include element i, (i) Pi represents the arrangement If Pi is preceded by the prefix i, then the full arrangement of n elements can be recursively defined as:
① If n=1, then the arrangement P has only one element i;
② If n>1, then the full arrangement P consists of the permutation (i)Pi;
According to the definition, it can be seen that if the permutation Pi of (k-1) elements has been generated, then the permutation of k elements can be generated by adding element i in front of each Pi .
Code Implementation

The code is as follows:

function rank($base, $temp=null) { $len = strlen($base); if($len <= 1) { echo $temp.$base.'
'; } else { for($i=0; $i< $len; ++$i) { rank(substr($base, 0, $i).substr($base, $i+1, $len-$i-1), $temp.$base[$i]); } } } rank('123');

However, after multiple test runs, it was found that there is a problem: if there are the same elements, the entire arrangement will be repeated.
For example, there are only three situations for the full arrangement of '122': '122', '212', and '221'; but the above method is repeated.
Slightly modified, added a flag to determine duplication, solved the problem (the code is as follows):

The code is as follows:

function fsRank($base, $temp=null) { static $ret = array(); $len = strlen($base); if($len <= 1) { //echo $temp.$base.'
'; $ret[] = $temp.$base; } else { for($i=0; $i< $len; ++$i) { $had_flag = false; for($j=0; $j<$i; ++$j) { if($base[$i] == $base[$j]) { $had_flag = true; break; } } if($had_flag) { continue; } fsRank(substr($base, 0, $i).substr($base, $i+1, $len-$i-1), $temp.$base[$i]); } } return $ret; } print '
'; print_r(fsRank('122')); print '
';
      
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