77. 组合

方法：递归

class Solution {
private:
    vector<vector<int>> res;
    vector<int> path;
    void solve(int n, int k, int idx) {
        if (path.size() == k) {
            res.push_back(path);
            return ;
        }

        for (int i = idx; i <= n - (k-path.size()) + 1; ++i) {
            path.push_back(i);
            solve(n, k, i + 1);
            path.pop_back();
        }
    }
public:
    vector<vector<int>> combine(int n, int k) {
        res.clear();
        solve(n, k, 1);
        return res;
    }
};

$时间复杂度O(),空间复杂度O(n+k);