39. Combination Sum

https://leetcode.com/problems/combination-sum/

Problem

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]
]

Constraints:

• 1 <= candidates.length <= 30

• 1 <= candidates[i] <= 200

• Each element of candidate is unique.

• 1 <= target <= 500

Solution

class Solution {
public:
    vector<vector<int>> combinationSum(vector<int>& cand, int target) {
        vector<vector<int>> combs;
        vector<int> comb;
        combinationSum(combs, comb, cand, 0, target);
        return combs;
    }
    void combinationSum(vector<vector<int>> &combs, vector<int> &comb, vector<int>& cand, int begin, int target) {
        if (target < 0) return;
        if (target == 0) combs.push_back(comb);
        for (int i = begin; i < cand.size(); ++i) {
            comb.push_back(cand[i]);
            combinationSum(combs, comb, cand, i, target - cand[i]);
            comb.pop_back();
        }
    }
};

• #dfs

• #backtracking

• #important