59.Spiral Matrix II

class Solution {
public:
    vector<vector<int> > generateMatrix(int n) {
        vector<vector<int> > res(n, vector<int>(n, 0));
        int val = 1, p = n;
        for (int i = 0; i < n / 2; ++i, p -= 2) {
            for (int col = i; col < i + p; ++col)
                res[i][col] = val++;
            for (int row = i + 1; row < i + p; ++row)
                res[row][i + p - 1] = val++;
            for (int col = i + p - 2; col >= i; --col)
                res[i + p - 1][col] = val++;
            for (int row = i + p - 2; row > i; --row)    
                res[row][i] = val++;
        }
        if (n % 2 != 0) res[n / 2][n / 2] = val;
        return res;
    }
};