1. use `int arr[][Columns]` for 2-d function declaration
2. `#pragma gcc optimize("Ofast, inline")` to increase speed
3. use parens to close bitwise operations and %
4. remember the scope in loops (int i) (int j)
5. `matrix[i][j]` in scanning a 2d array.
6. figure out `matrix[i][j]` and `matrix[i][n-1]`

More can be found in [[OJ-index]]