[leetcode] 77. Combinations

题目：

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

Example:

Input: n = 4, k = 2
Output:
[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

代码：

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int>> res;
        deal(res, vector<int>(), 1, k, n);
        return res;
    }
    
    void deal(vector<vector<int>>& res, vector<int> temp, int start, int k, int n){
        if(temp.size() == k){
            res.push_back(temp);
            return;
        }
        for(int i = start; i <= n; i++){
            temp.push_back(i);
            deal(res, temp, i+1, k, n);
            temp.pop_back();
        }
    }
};