Combinations

recursion

public class Solution {
    public ArrayList<ArrayList<Integer>> combine(int n, int k) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if(n < 1||k < 1)
            return null;
        ArrayList<Integer> list = new ArrayList<Integer>();
        generate(result, list, 1, n, k);
        return result;
    }
    
    private void generate(ArrayList<ArrayList<Integer>> result, ArrayList<Integer> list, int depth, int n, int k){
        if(list.size() == k){
            ArrayList<Integer> tmp = new ArrayList<Integer>();
            tmp.addAll(list);
            result.add(tmp);
            return;
        }
        
        for(int i=depth; i <= n; i++)
        {
            list.add(i);
            generate(result, list, i+1, n, k);
            list.remove(list.size()-1);
        }
    }
}