Question: 

Given n sets of choices: (1, 2, 3), (a, b, c), (i, ii, iii).

You pick one element from each set of choices. Generate all possible pickings. 

http://www.glassdoor.com/Interview/Given-n-sets-of-choices-1-2-3-2-3-4-4-5-You-pick-one-element-from-each-set-of-choices-Generate-all-possibl-QTN_218899.htm

private List
     
      
       > generateAllPickings(String[][] pool){    List
       
        
         > result = new ArrayList<>();    help(pool, 0, new ArrayList
         
          (), result);    return result;}private void help(String[][] pool,                   int group,                  List
          
            cur,                  List
           
            
             > result){    if (cur.size() == pool.length)    {        result.add(new ArrayList
             
              (cur));    }    if (group >= pool.length)    {        return;    }        // Now we are looking at pool[group], put each elements into the cur    for (int i = 0 ; i < pool[group].length ; i ++)    {        cur.add(pool[group][i]);        help(pool, group + 1, cur, result);        cur.remove(cur.size() - 1);    }}