3D Surface Area

• Time : O(nm)
• Space : O(1)

int surfaceArea(vector<vector<int>> A) {
    int res = 0, n = A.size(), m = A[0].size();
    int dy[4]{-1,0,1,0},dx[4]{0,1,0,-1};
    for(int i = 0; i < n; i++) {
        for(int j = 0; j < m; j++) {
            res += 2 + 4 * A[i][j];
            for(int k = 0; k < 4; k++) {
                int ny = i + dy[k], nx = j + dx[k];
                if(0 <= ny and ny < n and 0 <= nx and nx < m) {
                    res -= min(A[i][j], A[ny][nx]);
                }
            }
        }
    }
    return res;
}