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

思路：dfs

void dfs(vector<int> &candidates, int value, vector<int> temp, vector<vector<int> > &res, int idx) {
    if (value <= 0) {
        if (value == 0) {
            res.push_back(temp);
        }
        return;
    }
    //用i来控制无限制选取本数字，同时用i来控制不能选取本数字前面的数字，用来去重
    for (int i = idx; i < candidates.size(); i++) {
        temp.push_back(candidates[i]);
        dfs(candidates, value - candidates[i], temp, res, i);
        temp.pop_back();
    }
}

vector<vector<int> > combinationSum(vector<int>& candidates, int target) {
    vector<vector<int> > res;
    if (candidates.size() == 0) {
        return res;
    }
    vector<int> temp;
    dfs(candidates, target, temp, res, 0);
    return res;
}