LeetCode 77. Combinations

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

For example,
If n = 4 and k = 2, a solution is:

[
  [2,4],
  [3,4],
  [2,3],
  [1,2],
  [1,3],
  [1,4],
]

简单dfs：

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