39. Combination Sum

Given a set of candidate numbers (candidates) (without duplicates) and a target number (target), find all unique combinations in candidates where the candidate numbers sums to target.

The same repeated number may be chosen from candidates unlimited number of times.

Note:

• All numbers (including target) will be positive integers.

• The solution set must not contain duplicate combinations.

Example 1:

Input: candidates = [2,3,6,7], target = 7,
A solution set is:
[
  [7],
  [2,2,3]
]

Example 2:

Input: candidates = [2,3,5], target = 8,
A solution set is:
[
  [2,2,2,2],
  [2,3,3],
  [3,5]
]

vector<vector<int>> combinationSum(vector<int>& candidates, int target) { // time: O(2^n); space: O(2^n)
    vector<vector<int> > res;
    vector<int> cur;
    helper(candidates, res, cur, 0, target);
    return res;
}
void helper(vector<int>& candidates, vector<vector<int> >& res, vector<int>& cur, int start, int target) {
    if (target < 0) return;
    else if (target == 0) {
        res.push_back(cur);
    } else {
        for (int i = start; i < candidates.size(); ++i) {
            cur.push_back(candidates[i]);
            helper(candidates, res, cur, i, target - candidates[i]);
            cur.pop_back();
        }
    }
}