216. Combination Sum III

Find all possible combinations of k numbers that add up to a number n, given that only numbers from 1 to 9 can be used and each combination should be a unique set of numbers.

Example 1:

Input: k = 3, n = 7

Output:

[[1,2,4]]

Example 2:

Input: k = 3, n = 9

Output:

[[1,2,6], [1,3,5], [2,3,4]]

class Solution {
    public List<List<Integer>> combinationSum3(int k, int n) {  
        List<List<Integer>> res = new ArrayList<List<Integer>>();  
        if(k<1 || n<1) return res;  
        List<Integer> cur = new ArrayList<Integer>();  
        rec(res, cur, 0, k, n, 1);  
        return res;  
    }  

    private void rec(List<List<Integer>> res, List<Integer> cur, int sum, int k, int n, int level) {  
        if(sum==n && k==0) {  
            res.add(new ArrayList(cur));  
            return;  
        } else if(sum>n || k<=0) return;  

        for(int i=level; i<=9; i++) {  
            cur.add(i);  
            rec(res, cur, sum+i, k-1, n, i+1);  
            cur.remove(cur.size() - 1);  
        }  
    }  
}