题目：

给定一个二维网格和一个单词，找出该单词是否存在于网格中。
单词必须按照字母顺序，通过相邻的单元格内的字母构成，其中“相邻”单元格是那些水平相邻或垂直相邻的单元格。同一个单元格内的字母不允许被重复使用。
 
示例:
board =
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
]
给定 word = "ABCCED", 返回 true.
给定 word = "SEE", 返回 true.
给定 word = "ABCB", 返回 false.
 
 
代码：
class Solution(object):
    
    # 定义上下左右四个行走方向
    directs = [(0, 1), (0, -1), (1, 0), (-1, 0)]
    
    def exist(self, board, word):
        """
        :type board: List[List[str]]
        :type word: str
        :rtype: bool
        """
        m = len(board)
        if m == 0:
            return False
        n = len(board[0])
        mark = [[0 for _ in range(n)] for _ in range(m)]
                
        for i in range(len(board)):
            for j in range(len(board[0])):
                if board[i][j] == word[0]:
                    # 将该元素标记为已使用
                    mark[i][j] = 1
                    if self.backtrack(i, j, mark, board, word[1:]) == True:
                        return True
                    else:
                        # 回溯
                        mark[i][j] = 0
        return False
        
        
    def backtrack(self, i, j, mark, board, word):
        if len(word) == 0:
            return True
        
        for direct in self.directs:
            cur_i = i + direct[0]
            cur_j = j + direct[1]
            
            if cur_i >= 0 and cur_i < len(board) and cur_j >= 0 and cur_j < len(board[0]) and board[cur_i][cur_j] == word[0]:
                # 如果是已经使用过的元素，忽略
                if mark[cur_i][cur_j] == 1:
                    continue
                # 将该元素标记为已使用
                mark[cur_i][cur_j] = 1
                if self.backtrack(cur_i, cur_j, mark, board, word[1:]) == True:
                    return True
                else:
                    # 回溯
                    mark[cur_i][cur_j] = 0
        return False