Tic-Tac-Toe Winner

A tic-tac-toe board is an array<array<char, 3>, 3>: b[row][col] is 'X', 'O', or '.' for an empty cell.

Write a function char winner(const array<array<char, 3>, 3>& b) that returns 'X' if X has three in a line, 'O' if O has three in a line, and '-' if neither player has a line. A line is any of the 3 rows, 3 columns, or 2 diagonals. Three empty cells '.' in a line are NOT a win. Assume at most one player has a line.

The test prints the returned character on its own line. For the board
X X X
O O .
. . .

it prints the output shown in Expected Output.

Do NOT write a main function.