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)

class Solution {
public:
    vector<vector<int> > res;
    
    void permute(vector<int>& num, vector<int>& per, vector<bool>& isAvail) 
    {
        if(per.size() == num.size()) {
            res.push_back(per);
        }
        for(int i = 0; i < num.size(); i++) {
            if(isAvail[i]) {
                isAvail[i] = false;
                per.push_back(num[i]);
                permute(num, per, isAvail);
                isAvail[i] = true;
                per.pop_back();
            }
        }
    }
    
    vector<vector<int> > permute(vector<int> &num) {
        vector<int> per;
        vector<bool> isAvail(num.size(),true);
        permute(num, per, isAvail);
        return res;
    }
};