如何在Java中运行递归函数

Question

I am trying to learn Dynamic Programming and one of the examples they give in Wikipedia of what is not Dynamic Programming, is a recursive way of getting Fibonacci sequence up to certain number. IE

Given a recursive function, say:

fib(n) = 0 if n = 0
         1 if n = 1
         fib(n - 1) + fib(n - 2) if n >= 2

We can easily write this recursively from its mathematic form as:

function fib(n)
  if(n == 0 || n == 1)
    n
  else
    fib(n-1) + fib(n-2)

But I cannot get the pseudo code to work.

when I do this method in Java, I get an error the operator + is undefined for methods void :

 public void fib(int n) {

     if (n == 0 || n == 1) {

         System.out.println(n);
     } else
         return fib(n - 1) + fib(n - 2);

 }

Answer 1

in the line with return fib(n - 1) + fib(n - 2) , you add the return values of the fib() function. So you should actually return a (int) value, even if n==0||n==1.

 public int fib(int n) {

     if (n == 0 || n == 1) {
         return n;
     }
     else {
         return fib(n - 1) + fib(n - 2);
     }
 }

you then call and print you result from outside the function:

System.out.println(fib(42));

Answer 2

It has no return type so you can't mathematically add it to anything, let alone itself.

public void fib(int n)  //returning void

Give it a return type

public int fib(int n) //yey \o/

Answer 3

Try this.

public class Test
{
public static int fib( int n )
{
    if ( n == 0 || n == 1 )
        return n;
    else
        return fib( n - 1 ) + fib( n - 2 );
}

public static void main( String[] args )
{
    System.out.println(fib( 6 ));
}
}

Answer 4

Just change return type of fib function to int. and in function change

if (n == 0 || n == 1){return n; }