PHP recursion.

Just read the online php recursion, and asked the big God, told me that recursion is to call itself. Didn't understand. Also on Baidu saw the Hanoi tower problem. I know what happened to Hanoi, but I still don't understand recursion. Look at the cows to give a simple and understandable example ...

Reply to discussion (solution)

function f ($n)  {     $out =-1;   if ($n <0)       echo "Input cannot be negative";   else if ($n ==0| | $n ==1)       $out =1;   else $out =f ($n-1) * $n;     return $out;} Echo f (3);

The most classic example of factorial

function f ($n)  {     $out =-1;   if ($n <0)       echo "Input cannot be negative";   else if ($n ==0| | $n ==1)       $out =1;   else $out =f ($n-1) * $n;     return $out;} Echo f (3);

The most classic example factorial of this good ... Well, it looks like the IQ's not enough.

F (3) The parameter of the first pass to the F function is 3 run to $out=f ($n-1) * $n; It's changed. $OUT=F (3) = f (2) *3;f (2) The result has not yet been made, so the F function is called again, this time the F (2) parameter is 2 run again to $out=f ($n-1) * $n; when it becomes $out=f (2) = f (1) * *; F (1) When you run this function, you run the else if ($n ==0| | $n ==1) This code so f (1) = 1; When you know the result of f (1), we return the previous $out=f (2) = f (1) * = 1*1 =1; $out =f (2) = f (1) * = 1*2; $out =f (3) = f (2) * = 2*3; result is 6

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