输入一个字符串,按字典序打印出该字符串中字符的所有排列。
例如，输入字符串abc,则打印出由字符a,b,c所能排列出来的所有字符串abc,acb,bac,bca,cab和cba。
思路:
1.利用递归形成递归树,达到深度优先,固定首字母的效果
2.得复位以后才能再次深度优先
3.回溯法思想
4.一张图和一个运行过程,只能慢慢体会了
<?phpfunction test($str,$start,&$res){ //递归终止条件
if($start==strlen($str)){ $res[]=$str; return;
}
//
for($i=$start;$i<strlen($str);++$i){ if($i==$start || $str{$i}!=$str{$start}){
swap($str,$i,$start);var_dump($str.'==='.$start);var_dump($res);sleep(1);
test($str,$start+1,$res);
swap($str,$i,$start);
}
}
return $res;
}function swap(&$str,$a,$b){
if(!is_string($str)) return;
$t=$str{$a};
$str{$a}=$str{$b};
$str{$b}=$t;
}$str="abc";$res=array();//调用入口,从索引0开始
$res=test($str,0,$res);
var_dump($res);
string(7) "abc===0"array(0) {
}string(7) "abc===1"array(0) {
}string(7) "abc===2"array(0) {
}string(7) "acb===1"array(1) {
[0]=> string(3) "abc"}string(7) "acb===2"array(1) {
[0]=> string(3) "abc"}string(7) "bac===0"array(2) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"}string(7) "bac===1"array(2) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"}string(7) "bac===2"array(2) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"}string(7) "bca===1"array(3) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"}string(7) "bca===2"array(3) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"}string(7) "cba===0"array(4) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"
[3]=> string(3) "bca"}string(7) "cba===1"array(4) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"
[3]=> string(3) "bca"}string(7) "cba===2"array(4) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"
[3]=> string(3) "bca"}string(7) "cab===1"array(5) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"
[3]=> string(3) "bca"
[4]=> string(3) "cba"}string(7) "cab===2"array(5) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"
[3]=> string(3) "bca"
[4]=> string(3) "cba"}array(6) {
[0]=> string(3) "abc"
[1]=> string(3) "acb"
[2]=> string(3) "bac"
[3]=> string(3) "bca"
[4]=> string(3) "cba"
[5]=> string(3) "cab"}
