深度优先搜索（回溯）

class Solution {
public:
    vector<vector<int>>ans;
    vector<int>path;
    vector<vector<int>> combinationSum(vector<int>& nums, int target) {
        dfs(nums,0,target);
        return ans;
    }
    void dfs(vector<int>&nums,int u,int target){
        if(target==0){
            ans.push_back(path);
            return ;
        }
        if(u==nums.size())return ;
        for(int i=0;nums[u]*i<=target;i++){
            dfs(nums,u+1,target-nums[u]*i);
            path.push_back(nums[u]);
        }
        for(int i=0;nums[u]*i<=target;i++){
            path.pop_back();
        }
    }
};
//dfs