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MEDIUM

Combination Sum

Given an array of distinct integers candidates and a target integer target, return a list of all unique combinations of candidates where the chosen numbers sum to target.

You may return the combinations in any order.

The same number may be chosen from candidates an unlimited number of times. Two combinations are unique if the frequency of at least one of the chosen numbers is different.

Example

Input:

candidates = [2,3,6,7], target = 7

Output:

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

Explanation:
2 + 2 + 3 = 7 and 7 = 7.

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Constraints

• 1 ≤ candidates.length ≤ 30
• 2 ≤ candidates[i] ≤ 40
• All elements of candidates are distinct.
• 1 ≤ target ≤ 40

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Solution: Backtracking

• Time Complexity: O(2^t), where t = target.
• Space Complexity: O(t)

C++

class Solution {
public:
    vector<vector<int>> combinationSum(vector<int>& c, int target) {
        sort(c.begin(), c.end());
        vector<vector<int>> ans;
        vector<int> temp;
        std::function<void(int, int)> helper = [&](int index, int target) {
            if (target == 0) {
                ans.push_back(temp);
                return;
            }
            if (index >= c.size() || target < 0) return;
            helper(index+1, target);
            temp.push_back(c[index]);
            helper(index, target - c[index]);
            temp.pop_back();
        };
        helper(0, target);
        return ans;
    }
};