leetcode 216 - Combination Sum III

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. [1,2,1] is not valid because 1 is used twice.
Example 4:

Input: k = 3, n = 2
Output: []
Explanation: There are no valid combinations.
Example 5:

Input: k = 9, n = 45
Output: [[1,2,3,4,5,6,7,8,9]]
Explanation:
1 + 2 + 3 + 4 + 5 + 6 + 7 + 8 + 9 = 45
There are no other valid combinations.

Constraints:

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

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class Solution {
public:
vector<vector<int>> all_result;

vector<vector<int>> combinationSum3(int k, int n) {
vector<int> result;

combinationSum3(k, n, 1, result);
return all_result;
}

void combinationSum3(int k, int n, int i, vector<int> &result) {
if (n <= 0 || i > 9 || result.size() == k) {
if (n == 0 && result.size() == k)
all_result.push_back(result);
return;
}

for (int j = i; j <= 9; j++) {
result.push_back(j);
combinationSum3(k, n - j, j + 1, result);
result.pop_back();
}
}
};