algorithm - Prime factors using recursion in Java -

i'm having trouble recursion in java. have following method , should transform recursion without loop.

public static list<integer> primesloop(int n) {     list<integer> factors = new arraylist<integer>();     int f = 2;     while (f <= n)         if (n % f == 0) {             factors.add(f);             n /= f;         } else             f++;     return factors; } 

the recursive method should start same form:

public static list<integer> primesrec(int n); 

and should define methods transformation result example:

primesrec(900) -> prime factors of 900 : [2, 2, 3, 3, 5, 5] 

you can add f argument overloading, , adding private method take it, , invoked "main" public method.

in private method, have 3 cases:

1. stop clause: n==1: create new empty list
2. n%f == 0: recurse n'=n/f, f'=f, , add f list.
3. n%f != 0: recurse n'=n, f'=f+1, don't add list.

code:

public static list<integer> primesrecursive(int n) {     return primesrecursive(n,2);  }   //overlaod private method takes f argument:  private static list<integer> primesrecursive(int n, int f) {      if (n == 1) return new arraylist<integer>();      if (n % f == 0) {          list<integer> factors = primesrecursive(n/f, f);          factors.add(f);          return factors;      } else          return primesrecursive(n,f+1);  } 

as expected, invoking:

public static void main(string args[]) {     system.out.println(primesrecursive(900)); } 

will yield:

[5, 5, 3, 3, 2, 2] 

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note: if want factors in ascending order:

1. switch arraylist implementation linkedlist in stop clause (for performance issues)
2. add items factors.add(0,f); instead factors.add(f)