为什么递归数值方法会输出其输出

Question

Can anyone tell me in the simplest way why this prints out 321123 when generate(123) is called? I understand why it prints out 321, but the 123 part eludes me.

 /** @param x an integer such that x >= 0
 */ 

public void generate(int x) {     

     System.out.print(x % 10);     

     if ((x / 10) != 0) {         
        generate(x / 10);     
     }     
     System.out.print(x % 10); 
}

Answer 1

The recursion goes "in", then comes back "out". Think of it this way:

generate(123) {
    System.out.print(123 % 10)          // 3

    generate(12) {
        System.out.print(12 % 10)       // 2

        generate(1) {
            System.out.print(1 % 10)    // 1
            System.out.print(1 % 10)    // 1
        }

        System.out.print(12 % 10)       // 2
    }

    System.out.print(123 % 10)          // 3
}

Answer 2

Because you print x%10 both before and after your recursive call. When the number is 123, it prints 3, does the work for 12, then prints 3 again.

When it gets 12, it prints 2, does the work for 1, then prints 2 again.

Same for the 1. So you get "nested" prints of the remainder in each call.

Answer 3

Entrance to deeper function levels:

if ((x / 10) != 0) {
generate(x / 10);
}

is also exit with the filo order which reverses 321 to 123

becasue there is another

System.out.print(x % 10);

after that.