lintcode-medium-Combinations

public class Solution {
    /**
     * @param n: Given the range of numbers
     * @param k: Given the numbers of combinations
     * @return: All the combinations of k numbers out of 1..n
     */
    public List<List<Integer>> combine(int n, int k) {
        // write your code here
        
        List<List<Integer>> result = new ArrayList<List<Integer>>();
        
        if(n < 1 || k > n)
            return result;
        
        List<Integer> line = new ArrayList<Integer>();
        
        helper(result, line, n, k, 1);
        
        return result;
    }
    
    public void helper(List<List<Integer>> result, List<Integer> line, int n, int k, int start){
        
        if(line.size() == k){
            result.add(new ArrayList<Integer>(line));
            return;
        }
        
        for(int i = start; i <= n; i++){
            line.add(i);
            helper(result, line, n, k, i + 1);
            line.remove(line.size() - 1);
        }
        
        return;
    }
    
}