216. Combination Sum III

Problem Statement

Medium

Find all valid combinations of k numbers that sum up to n such that the following conditions are true:

Only numbers 1 through 9 are used.
Each number is used at most once.

Return a list of all possible valid combinations. The list must not contain the same combination twice, and the combinations may be returned in any order.

Example 1:

Input: k = 3, n = 7
Output: [[1,2,4]]
Explanation:
1 + 2 + 4 = 7
There are no other valid combinations.

Example 2:

Input: k = 3, n = 9
Output: [[1,2,6],[1,3,5],[2,3,4]]
Explanation:
1 + 2 + 6 = 9
1 + 3 + 5 = 9
2 + 3 + 4 = 9
There are no other valid combinations.

Example 3:

Input: k = 4, n = 1
Output: []
Explanation: There are no valid combinations.
Using 4 different numbers in the range [1,9], the smallest sum we can get is 1+2+3+4 = 10 and since 10 > 1, there are no valid combination.

Constraints:

2 <= k <= 9
1 <= n <= 60

Java

Source file

class Solution {
    List<List<Integer>> ans = new ArrayList<>();

    public List<List<Integer>> combinationSum3(int k, int n) {
        solve(k, n, new ArrayList<>(), 1);
        return ans;
    }

    void solve(int k, int n, List<Integer> curr, int digit) {
        if (curr.size() == k) {
            if (n == 0)
                ans.add(new ArrayList<>(curr));
            return;
        }
        for (int i = digit; i < 10; i++) {
            if (n - i < 0)
                break;
            curr.add(i);
            solve(k, n - i, curr, i + 1);
            curr.remove(curr.size() - 1);
        }
    }
}