LeetCode Combinations

class Solution {

private:

    vector<vector<int> > res;

public:

    vector<vector<int> > combine(int n, int k) {

        res.clear();

        vector<int> path;

        dfs(1, n, k, path);

        return res;

    }

    

    void dfs(int lower, int upper, int level, vector<int>& path) {

        if (level == 0) {

            res.push_back(path);

            return;

        }

        for (int i=lower; i<= upper - level + 1; i++) {

            path.push_back(i);

            dfs(i + 1, upper, level -1, path);

            path.pop_back();

        }

    }

};

常规dfs