Permutations I

Given a collection of numbers, return all possible permutations.

For example,
[1,2,3] have the following permutations:
[1,2,3], [1,3,2], [2,1,3], [2,3,1], [3,1,2], and [3,2,1].

Solution: dfs(recursion)

 1 class Solution {
 2 public:
 3     vector<vector<int> > res;
 4     
 5     void permute(vector<int>& num, vector<int>& per, vector<bool>& isAvail) 
 6     {
 7         if(per.size() == num.size()) {
 8             res.push_back(per);
 9         }
10         for(int i = 0; i < num.size(); i++) {
11             if(isAvail[i]) {
12                 isAvail[i] = false;
13                 per.push_back(num[i]);
14                 permute(num, per, isAvail);
15                 isAvail[i] = true;
16                 per.pop_back();
17             }
18         }
19     }
20     
21     vector<vector<int> > permute(vector<int> &num) {
22         vector<int> per;
23         vector<bool> isAvail(num.size(),true);
24         permute(num, per, isAvail);
25         return res;
26     }
27 };