# Xs and Os Referee

Elementary+
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Tic-Tac-Toe, sometimes also known as Xs and Os, is a game for two players (X and O) who take turns marking the spaces in a 3×3 grid. The player who succeeds in placing three respective marks in a horizontal, vertical, or diagonal rows (NW-SE and NE-SW) wins the game.

But we will not be playing this game. You will be the referee for this games results. You are given a result of a game and you must determine if the game ends in a win or a draw as well as who will be the winner. Make sure to return "X" if the X-player wins and "O" if the O-player wins. If the game is a draw, return "D".

A game's result is presented as a list of strings, where "X" and "O" are players' marks and "." is the empty cell.

Input: A game result as list of strings (unicode).

Output: "X", "O" or "D" as a string.

Examples:

```assert checkio(["X.O", "XX.", "XOO"]) == "X"
assert checkio(["OO.", "XOX", "XOX"]) == "O"
assert checkio(["OOX", "XXO", "OXX"]) == "D"
assert checkio(["O.X", "XX.", "XOO"]) == "X"
```

How it is used: The concepts in this task will help you when iterating data types. They can also be used in game algorithms, allowing you to know how to check results.

Preconditions:

• there is either one winner or a draw;
• len(game_result) == 3;
• all(len(row) == 3 for row in game_result).

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