[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。

 1 class Solution {
 2 public:
 3     void dfs(int n,int k,int start,vector<int>&tmp,vector<vector<int>>& result)
 4     {
 5         if(tmp.size()==k)
 6         {
 7             result.push_back(tmp);
 8         }
 9         for(int i=start;i<=n;i++)
10         {
11             tmp.push_back(i);
12             dfs(n,k,i+1,tmp,result);
13             tmp.pop_back();
14         }
15     }
16     vector<vector<int>> combine(int n, int k) {
17         vector<int> tmp;
18         vector<vector<int>> result;
19         if(n<1 ||(k>n)) return result;
20         dfs(n,k,1,tmp,result);
21         return result;
22     }
23 };