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 ]
]

class Solution {
public:
    vector<vector<int> > generateMatrix(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        vector<vector<int> > result(n, vector<int>(n));
        if (n == 0) return result;
        
        int num = 0;
        int layer = n/2;
        for (int i = 0; i < layer; ++i)
        {
            int j;
            for (j = i; j < n-1-i; ++j)
                result[i][j] = ++num;
            for (j = i; j < n-1-i; ++j)
                result[j][n-1-i] = ++num;
            for (j = n-1-i; j > i; --j)
                result[n-1-i][j] = ++num;
            for (j = n-1-i; j > i; --j)
                result[j][i] = ++num;
        }
        if (n%2 == 1)
            result[n/2][n/2] = ++num;
        return result;
    }
};