leetcode 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]
]

思路：回溯法

class Solution {
    void combinationSum1(vector<int> candidates, int target, vector<vector<int> > &res, vector<int> &v, int len, int begin) {
        if (target == 0) {
            res.push_back(v);
            return ;
        }
        for (int i = begin; (i < len) && target >= candidates[i]; i++) {
            v.push_back(candidates[i]);
            combinationSum1(candidates, target - candidates[i], res, v, len, i);
            v.pop_back();
        }
        
    }
public:
    vector<vector<int>> combinationSum(vector<int>& candidates, int target) {
        sort(candidates.begin(), candidates.end());
        vector<vector<int> > res; 
        vector<int> v;
        int len = candidates.size();
        combinationSum1(candidates, target, res, v, len, 0);
        return res;
    }
};