递归函数

纠结了2天的递归函数，记录一下

<php>
function sum($n,$m){
    if($m<=$n){
        return $n;
    }
    return sum($n,$m-1)+$m;
}
echo sum(1,3);  // => 6
</php>

推算一下运算过程

sum(1,3) = sum(1,2) + 3 , sum(1,2) = sum(1,1) + 2 , sum(1,1) =1

得到了sum(1,1) =1 后，再进行倒推

sum(1,1) =1 , sum(1,2) = 1 + 2 = 3 , sum(1,3) = 3+3=6