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这个递归是如何运算的

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Release: 2016-06-06 20:32:41
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<code><?php $string = 'abc';
reverse($string);
function reverse($str){
    if(strlen($str)>0){
        reverse(substr($str,1));
    }
    echo substr($str,0,1);
    return;
}
?>
</code>
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这是将abc顺序颠倒显示cba的递归函数。函数执行时每一步是怎样执行的才得到最后颠倒的顺序。请分解一下执行步骤,谢谢

回复内容:

<code><?php $string = 'abc';
reverse($string);
function reverse($str){
    if(strlen($str)>0){
        reverse(substr($str,1));
    }
    echo substr($str,0,1);
    return;
}
?>
</code>
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这是将abc顺序颠倒显示cba的递归函数。函数执行时每一步是怎样执行的才得到最后颠倒的顺序。请分解一下执行步骤,谢谢

<code>php</code><code>$string = 'abc';
reverse($string);
function reverse($str){
    if(strlen($str)>0){
        reverse(substr($str,1));
    }
    echo substr($str,0,1);//用var_dump(substr($str,0,1));看
    return;
}
第一次调用reverse(abc) if成立 
再二次调用reverse(bc) if成立
再三次调用reverse(c) if成立
再四次调用reverse('') if不成立
然后往回调,依次输出false,c,b,a
其实可以implode('',array_reverse(str_split('abc')));//cba
</code>
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