LeetCode Combination Sum

Given a set of candidate numbers (C) 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.
• Elements in a combination (a1, a2, … , ak) must be in non-descending order. (ie, a1 ≤ a2 ≤ … ≤ ak).
• 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] 

题意：从数组中找到和为目标的组合，每个数允许重复。

思路：排序后，利用回溯去做，为了让每个数都能重复，所以枚举的时候可以在从这个数的下标开始一次

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