Find Word in Grid

This problem was asked by Coursera.

Given a 2D board of characters and a word, find if the word exists in the grid.

The word can be constructed from letters of sequentially adjacent cell, where “adjacent” cells are those horizontally or vertically neighboring. The same letter cell may not be used more than once.

For example, given the following board:

[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]

exists(board, "ABCCED") returns true, exists(board, "SEE") returns true, exists(board, "ABCB") returns false.

My Solution(Python):

class Node:
    def __init__(self, char, neighbours, visited=False):
        self.char = char
        self.neighbours = neighbours
        self.visited = visited


def get_nodes(board):
    nodes = []
    n_rows, n_cols = len(board), len(board[0])
    for i in range(n_rows):
        for j in range(n_cols):
            neighbours = []
            if i==0:
                neighbours.append(board[i+1][j])
            elif i==n_rows-1:
                neighbours.append(board[i-1][j])
            elif i>0 and i<n_rows-1:
                neighbours.append(board[i-1][j])
                neighbours.append(board[i+1][j])
            if j==0:
                neighbours.append(board[i][j+1])
            elif j==n_cols-1:
                neighbours.append(board[i][j-1])
            elif j>0 and j<n_cols-1:
                neighbours.append(board[i][j-1])
                neighbours.append(board[i][j+1])
            node = Node(char=board[i][j], neighbours=neighbours, visited=False)
            nodes.append(node)
    return nodes


def exists(board, word):
    word = list(word)
    nodes = get_nodes(board)
    while word:
        found = False
        letter = word.pop(0)
        if not word:
            for node in nodes:
                if node.char == letter and node.visited == False:
                    return True
                else:
                    continue
            return False
        next_letter = word[0]
        # print(letter, next_letter)
        for node in nodes:
            if node.char == letter and next_letter in node.neighbours and node.visited == False:
                found = True
                node.visited = True
                break
            else:
                continue
        if not found:
            return False

def main():
    board = [['A','B','C','E'],
             ['S','F','C','S'],
             ['A','D','E','E']
            ]
    print(exists(board, "ABCCED"))
    print(exists(board, "SEE"))
    print(exists(board, "ABCB"))


if __name__=='__main__':
    main()