[LeetCode]Combinations

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],
]

 

深搜，递归，回溯。这是一类题，举一反三。

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