[LeetCode] Spiral Matrix II

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 ]
]

class Solution {
public:
    vector<vector<int> > generateMatrix(int n) {
        if (n == 0) return vector<vector<int> >();
        static const int dy[] = {0,1,0,-1}; // right, down, left, up
        static const int dx[] = {1,0,-1,0};
        vector<vector<int> > res;
        res.resize(n);
        for (int i = 0; i < n; ++i) res[i].resize(n,0);
        int cur = 1;
        for (int i = 0; i < n; ++i) res[0][i] = cur++;
        for (int i = 1; i < n; ++i) res[i][n-1] = cur++;
        for (int i = n-2; i >= 0; --i) res[n-1][i] = cur++;
        
        int y = n-1, x = 0, dir = 3;
        int t = n*n;
        while (cur <= t) {
            if (res[y+dy[dir%4]][x+dx[dir%4]] != 0) {
                dir++; continue;
            }
            y += dy[dir%4];
            x += dx[dir%4];
            res[y][x] = cur++;
        }
        return res;
    }
};