[Leetcode] Permutations

class Solution {
private:
    vector<vector<int> > res;

public:
    vector<vector<int> > permute(vector<int> &num) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        res.clear();
        run(0, num);
        return res;
    }
    
    void run(int startIdx, vector<int> num)
    {
        if (startIdx == num.size())
        {
            res.push_back(num);
        }
        
        for (int i = startIdx; i < num.size(); ++i)
        {
            swap(num[startIdx], num[i]);
            run(startIdx + 1, num);
            swap(num[startIdx], num[i]);
        }
    }
    
    void swap(int& a, int& b)
    {
        if (a == b)
            return;
            
        a = a ^ b;
        b = a ^ b;
        a = a ^ b;
    }
};