Leetcode: 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]]

Credits:

Special thanks to
@mithmatt for adding this problem and creating all test cases.

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class Solution {
private:
int index_count;
vector<vector<int> > results;
public:
void backtrace(int target, int sum, vector<int>& index, int id, int n, int k)
{
if (sum > target || n > k) {
return;
}
else if (sum == target && n == k) {
vector<int> result;
for(int i = 1; i <= n; ++i)
{
result.push_back(index[i]);
}
results.push_back(result);
return;
}

for (int i = id + 1; i < 10; ++i)
{
index[n+1] = i;
backtrace(target, sum + i, index, i, n+1, k);
}
}
vector<vector<int>> combinationSum3(int k, int n) {
if (k * 9 < n) {
return results;
}

vector<int> index = vector<int>(10, 0);
for (int i = 0; i < 10; ++i) {
index[i] = i;
}
results.clear();
backtrace(n, 0, index, 0, 0, k);

return results;

}
};