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 [[CS-index]]