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],
]

解法：dfs基础题

class Solution {
public:
    
    vector<vector<int> > combine(int n, int k) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int>> ret;
        vector<int> vec(k,0);
        dfs(ret,vec,0,1,n,k);
        return ret;
    }

    void dfs(vector<vector<int>> &ret,vector<int> vec,int dep,int start,int n,int k)
    {
        if(dep == k){
            ret.push_back(vec);
            return;
        }
        for(int i = start;i <= n;i++)
        {
            vec[dep] = i;
            dfs(ret,vec,dep+1,i+1,n,k);
        }
    }
};

72 milli secs.