leetcode 77. 组合

给定两个整数 n 和 k，返回 1 ... n 中所有可能的 k 个数的组合。
示例:
输入: n = 4, k = 2
输出:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        vector<int> out;
        dfs(n, k, 1, 0, out, res);
        return res;       
    }
    void dfs(int n, int k, int start, int count, vector<int>& out, vector<vector<int>>& res) {
        if (count == k) {
            res.push_back(out);
            return;
        }
        for (int i = start; i <= n; i++) {
            out.push_back(i);
            dfs(n, k, i + 1, ++count, out, res);
            out.pop_back();
            count--;
        }      
    }
};