# C# Solution

• ``````public class Solution {
public void Solve(char[,] board) {
bool[,] visited = new bool[board.GetLength(0), board.GetLength(1)];

for (int i = 0; i <= board.GetLength(1) - 1; i++)
if (board[0, i] == 'O' && !visited[0, i])
MarkInvalid(0, i, board, visited);

for (int i = 0; i <= board.GetLength(1) - 1; i++)
if (board[board.GetLength(0) - 1, i] == 'O' && !visited[board.GetLength(0) - 1, i])
MarkInvalid(board.GetLength(0) - 1, i, board, visited);

for (int i = 0; i <= board.GetLength(0) - 1; i++)
if (board[i, 0] == 'O' && !visited[i, 0])
MarkInvalid(i, 0, board, visited);

for (int i = 0; i <= board.GetLength(0) - 1; i++)
if (board[i, board.GetLength(1) - 1] == 'O' && !visited[i, board.GetLength(1) - 1])
MarkInvalid(i, board.GetLength(1) - 1, board, visited);

for (int i = 0; i <= board.GetLength(0) - 1; i++)
for (int j = 0; j <= board.GetLength(1) - 1; j++)
if (board[i, j] == '-')
board[i, j] = 'O';
else
board[i, j] = 'X';
}

private void MarkInvalid(int x, int y, char[,] board, bool[,] visited)
{
visited[x, y] = true;
board[x, y] = '-';

if (x - 1 > 0 && board[x - 1, y] != 'X' && !visited[x - 1, y])
MarkInvalid(x - 1, y, board, visited);
if (x + 1 < board.GetLength(0) - 1 && board[x + 1, y] != 'X' && !visited[x + 1, y])
MarkInvalid(x + 1, y, board, visited);
if (y - 1 > 0 && board[x, y - 1] != 'X' && !visited[x, y - 1])
MarkInvalid(x, y - 1, board, visited);
if (y + 1 < board.GetLength(1) && board[x, y + 1] != 'X' && !visited[x, y + 1])
MarkInvalid(x, y + 1, board, visited);
}
}``````

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