leetcode 79. 单词搜索

'''
leetcode 79. 单词搜索
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。

递归+回溯
'''
class Solution:
    def exist(self, board, word: str) -> bool:
        if len(word)==0:
            return True
        if len(board)==0:
            return False
        flag_array=[[False for a in range(len(board[0]))] for b in range(len(board))]
        # print(flag_array)
        for i in range(len(board)):# 逐行扫描
            for j in range(len(board[0])):#逐列扫描
                if board[i][j]==word[0]:
                    flag_array[i][j]=True
                    if self.helper(i,j,board,word[1:],flag_array):
                        return True
                    else:
                        flag_array[i][j] = False
        return False
    def helper(self,start_i,start_j,board,word,flag_array):
        '''
        已经找到当前字母的字符串，下一个待寻找的字母是word[0]
        上一个被找到的前一个字符在二维列表中的索引值 i,j
        flag_array 表示当前递归算法能够搜索的区间范围
        :param i:
        :param j:
        :param board:
        :param word:
        :param flag_array
        :return:
        '''
        if len(word)==0:
            return True
        output=False
        for p in range(-1,2,2):# 列数不变，搜索前后的行数
            if not((start_i + p) >= 0 and (start_i + p) < len(board)):
                continue
            else:
                if board[start_i + p][start_j] == word[0] and not flag_array[start_i + p][start_j]:
                    flag_array[start_i + p][start_j] = True
                    if self.helper(start_i+p,start_j,board,word[1:],flag_array):
                        return True
                    else:
                        flag_array[start_i + p][start_j] = False

        for q in range(-1,2,2):#行数不变，搜索当前的列数
             # print(p,q)
             if (start_j+q)>=0 and (start_j+q)<len(board[0]) and not flag_array[start_i][start_j+q]:
                if board[start_i][start_j+q]==word[0]:
                    flag_array[start_i][start_j+q]=True
                    if self.helper(start_i,start_j+q,board,word[1:],flag_array):
                        return True
                    else:
                        flag_array[start_i][start_j + q] = False
        return False

if __name__=="__main__":
    print(Solution().exist(board =
[
  ['A','B','C','E'],
  ['S','F','C','S'],
  ['A','D','E','E']
],
        word='ABCCED'
))