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);          }      }  }