[LeetCode] Combinations—

[LeetCode] Combinations——递归

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],
]

要求

给定两个整数（n,k）,返回长度为k，从1到n的所有组合（注意1.组合没有顺序 2. [2，3]和[3，2]为同一组，取前者小于后者的那一组）。

 1 class Solution {
 2 public:
 3     vector<vector<int> > combine(int n, int k) {
 4         v.resize(k);
 5         build(0,n,k,1);
 6         return res;
 7     }
 8     void build(int dep, int n, int k,int start){
 9         if(dep==k){ 
10             res.push_back(v);
11             return;
12         }
13         for(int i=start;i<=n;i++){
14             v[dep]=i;
15             build(dep+1,n,k,i+1);
16         }
17         
18     }
19     vector<int> v;
20     vector<vector<int>> res;
21 };