陷入递归

Question

通过Java完整参考我陷入了递归。我打印中间结果以查看值的变化。我可以看到num的值将递减到1并返回1，之后num的值如何递增到3和4？有什么帮助吗？

class Recursion {

    public  int fact(int num){
        int result;

        if(num==1){
            return 1;
        }

        System.out.println("Befor fact value of num is: "+num);//here ok!
        result=fact(num-1)*num;
        System.out.println("After value of num is: "+num); //here not ok!
        //System.out.println("result= "+result);
        return result;
    }
    public static void main(String[]args){
        Recursion obj=new Recursion();
        System.out.println(obj.fact(4));
    }
}

Answer 1

您似乎正在打印错误的变量，

System.out.println("Before fact value of n is: "+num);//here ok!
result=fact(num-1)*num;
// You didn't change "num".
System.out.println("After value of n is: "+result); //result, not num.
//System.out.println("result= "+result);
return result;

打印

Before fact value of num is: 4
Before fact value of num is: 3
Before fact value of num is: 2
After value of num is: 2
After value of num is: 6
After value of num is: 24
24

Answer 2

每次调用一个事实方法时，Java都会将该调用保存在堆栈中，它也会保存num变量的值，因为它是事实方法的局部变量。

由于您未更改num值，因此发生的情况将如下：

call fact(4) > print: Befor fact value of n is: 4
Then call fact(3) > print: Befor fact value of n is: 3
Then call fact(2) > print: Befor fact value of n is: 2

并且自从Stack最后出现之后，发生的事情就像是：

continue with call of fact(2) > print: After value of n is: 2
Then continue with call of fact(3) > print: After value of n is: 3
Then continue with first call of fact(4) > print: After value of n is: 4