Php_ Dictionary ordering method to get permutations and combinations

A time before a party chatting to a problem, is to give you a row of arrays, such as 1,2,3,4,5, how to efficiently get all the permutations of the above series, happened to be nothing, studied a bit, at first thought is a very simple problem, began to write code directly, and then found how the cycle is not ideal, Basically have some unnecessary consumption, Baidu see a good algorithm, dictionary order method, by the way to learn, and then record it.

Picking an algorithm idea:

Set P is a full arrangement of [1,n].

P=p1p2 ... Pn=p1p2 ... Pj-1pjpj+1 ... Pk-1pkpk+1 ... Pn, j=max{i| Pi<pi+1}, k=max{i| PI>PJ}, swap pj,pk, will pj+1 ... Pk-1pjpk+1 ... PN Flip, P ' = p1p2 ... PJ-1PKPN ... Pk+1pjpk-1 ... Pj+1 is the next

example: The next permutation of 839647521.

Starting from the right, Find the first number 4 smaller than the right (because 4<7, and 7>5>2>1), and then from the far right, find 4 to the right than 4 of the number 5 (because 4>2>1 and 4<5), Exchange 4, 5, at this time 5 to the right 7421, upside down to 1247, That is the next arrangement: 839651247. Use this method to write out a fully arranged non-recursive calculation

Feeling is quite clear, on the paper a little bit, and then a little verification, really good, and then the implementation of their own PHP a bit:

/*   字典序法获取所有排列   @最后更新时间：03/05/2014   @toryzen(toryzen.com)   @备注，备注中示例用ABC顺序表示 */function getPars( $arr ){      //正向排列      sort( $arr );      //获取数组长度      $len = count ( $arr )-1;      //记录传入的排列      $return [] = $arr ;      while (TRUE){          //从右侧开始找到第一个左侧(A)<右侧(B)的数字序列          for ( $i = $len ; $i >=0; $i --){              if ( $arr [ $i ]> $arr [ $i -1]){                  $here = $i -1;                  break ;              }          }          //若找到了则开始换位          if ( $here >=0){              //从有右向左侧找，第一个比左侧(A)大的数字(C)交换位置得到CBA              for ( $j = $len ; $j > $here ; $j --){                  if ( $arr [ $here ]< $arr [ $j ]){                      $revers = $j ;                      list( $arr [ $here ], $arr [ $j ]) = array ( $arr [ $j ], $arr [ $here ]);                      break ;                  }              }              //将后续数字倒序得到CAB              unset( $newarr );              for ( $h = $here +1; $h <= $len ; $h ++){                  $newarr [] = $arr [ $h ];                  unset( $arr [ $h ]);              }              $return [] = $arr = array_merge ( $arr , array_reverse ( $newarr ,TRUE));          } else {              break ;          }     }      return $return ;}$arr = array (1,4,3,2);print_r(getPars( $arr ));

In addition to this there is a recursive method, but their own recursive learning is not very good, and all the recursion can be used to solve the cycle with the loop, a day, there is time, and then study recursion, always turn to bend.

Php_ Dictionary ordering method to get permutations and combinations