[LeetCode] Spiral Matrix II

Given an integer n, generate a square matrix filled with elements from 1 to n2 in spiral order.

For example,
Given n = 3,

You should return the following matrix:

[
 [ 1, 2, 3 ],
 [ 8, 9, 4 ],
 [ 7, 6, 5 ]
]

Solution:

class Solution {
public:
    int **used;
    vector<vector<int> > generateMatrix(int n) {
        vector< vector<int > > ans;
        if(n == 0) return ans;
        
        used = new int*[n];
        for(int i = 0;i < n;i++)
        {
            used[i] = new int[n];
            memset(used[i], 0, n * sizeof(int));
        }
        
        int x = 0, y = -1, count = 1, dir = 1; //1 for right, 2 for down, 3 for left, 4 for up
        bool flag = true;
        while(flag)
        {
            switch(dir)
            {
                case 1:
                //current direction is right, try continue
                if(y + 1 < n && used[x][y + 1] == 0)
                    used[x][(y++) + 1] = count++;
                    else if(x + 1 < n && used[x + 1][y] == 0)
                        {
                            used[(x++) + 1][y] = count++;
                            dir = 2;
                        }
                        else
                            flag = false;   
                break;
                case 2:
                //current direction is right, try continue
                if(x + 1 < n && used[x + 1][y] == 0)
                    used[(x++) + 1][y] = count++;
                    else if(y - 1 >= 0 && used[x][y - 1] == 0)
                        {
                            used[x][(y--) - 1] = count++;
                            dir = 3;
                        }
                        else
                            flag = false;   
                break;
                case 3:
                //current direction is right, try continue
                if(y - 1 >= 0 && used[x][y - 1] == 0)
                    used[x][(y--) - 1] = count++;
                    else if(x - 1 >= 0 && used[x - 1][y] == 0)
                        {
                            used[(x--) - 1][y] = count++;
                            dir = 4;
                        }
                        else
                            flag = false;
                break;
                case 4:
                //current direction is right, try continue
                if(x - 1 >= 0 && used[x - 1][y] == 0)
                    used[(x--) - 1][y] = count++;
                    else if(y + 1 < n && used[x][y + 1] == 0)
                        {
                            used[x][(y++) + 1] = count++;
                            dir = 1;
                        }
                        else
                            flag = false;   
                break;
                default: break;
            }
        }
        
        for(int i = 0;i < n;i++)
        {
            vector<int > tmp;
            for(int j = 0;j < n;j++)
                tmp.push_back(used[i][j]);
            ans.push_back(tmp);
        }
        return ans;
    }
};