Combinations

06/28/2016 Backtracking

Question

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],
]

Solution

Result: Accepted Time: 12 ms

Here should be some explanations.

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

Complexity Analytics

  • Time Complexity:
  • Space Complexity: ?