39. Combination Sum (Medium)

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

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

Note:

• All numbers (including target) will be positive integers.
• The solution set must not contain duplicate combinations.

For example, given candidate set [2, 3, 6, 7] and target 7, A solution set is:

[
  [7],
  [2, 2, 3]
]

Solution: DFS

class Solution {
    void dfs(vector<int>& candidates, int target, vector<int>& out, int pos, vector<vector<int>>& res) {
        if (target == 0) {
            res.push_back(out);
            return;
        }
        for (int i = pos; i < candidates.size() && candidates[i] <= target; ++i) {
            out.push_back(candidates[i]);
            dfs(candidates, target-candidates[i], out, i, res);
            out.pop_back();
        }
    }
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        vector<vector<int>> res;
        vector<int> out;
        sort(candidates.begin(), candidates.end());
        dfs(candidates, target, out, 0, res);
        return res;
    }
};