77. Combinations

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],
]
C++：

class Solution {
public:
    vector<vector<int>> combine(int n, int k) {
        vector<vector<int> > result;
        vector<int> path;
        vector<int> nums;
        for (int i = 1; i <= n; i++){
            nums.push_back(i);
        }
        dfs(nums, 0, k, path, result);
        return result;
    }
    
    void dfs(vector<int>nums, int start, int k, vector<int>& path, vector<vector<int> >& result){
        if (path.size() == k){
            result.push_back(path);
            return;
        }
        for (int i = start; i < nums.size(); i++){
            path.push_back(nums[i]);
            dfs(nums, i + 1, k, path, result);
            path.pop_back();
        }
    }    
};