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.
Note:
All numbers will be positive integers.
The solution set must not contain duplicate combinations.
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]]
vector<vector<int>> combinationSum3(int k, int n) { // time: O(C(9, k)); space: O(C(9, k))
vector<vector<int> > res;
vector<int> cur;
helper(res, cur, k, n, 1);
return res;
}
void helper(vector<vector<int> >& res, vector<int>& cur, int k, int n, int start) {
if (k == 0 && n == 0) {
res.push_back(cur);
} else if (k == 0 || n == 0) {
return;
} else {
for (int i = start; i <= 9 && i <= n; ++i) {
cur.push_back(i);
helper(res, cur, k - 1, n - i, i + 1);
cur.pop_back();
}
}
}
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