replace O's with X's

class Solution:
    def fill(self, n, m, mat):
        # Use the input matrix itself as the visited array
        # Mark all 'O' cells that are on the border of the matrix with '#'
        for i in range(n):
            if mat[i][0] == 'O':
                mat[i][0] = '#'
            if mat[i][m - 1] == 'O':
                mat[i][m - 1] = '#'
        for j in range(m):
            if mat[0][j] == 'O':
                mat[0][j] = '#'
            if mat[n - 1][j] == 'O':
                mat[n - 1][j] = '#'

        # Use a BFS approach to mark all 'O' cells that are connected to the border cells with '#'
        queue = []
        for i in range(n):
            for j in range(m):
                if mat[i][j] == '#':
                    queue.append((i, j))

        while queue:
            i, j = queue.pop(0)

            for di, dj in [(-1, 0), (1, 0), (0, -1), (0, 1)]:
                new_i = i + di
                new_j = j + dj
                if 0 <= new_i < n and 0 <= new_j < m and mat[new_i][new_j] == 'O':
                    mat[new_i][new_j] = '#'
                    queue.append((new_i, new_j))

        # Replace all 'O' cells that are not marked with 'X'
        # Restore the marked cells back to 'O'
        for i in range(n):
            for j in range(m):
                if mat[i][j] == '#':
                    mat[i][j] = 'O'
                else:
                    mat[i][j] = 'X'

        return mat