79. Word Search

79. Word Search

import numpy as np

class Solution:
    def exist(self, board: List[List[str]], word: str) -> bool:
        m,n=len(board),len(board[0])
        dic=defaultdict(set)
        for i in range(m):
            for j in range(n):
                dic[board[i][j]].add((i,j))

        def dfs(cord,indx):
            if indx==lngth:
                # if word_isr:results.append(word[::-1])
                # else:results.append(word)
                return True
 
            ch=word[indx]
            i,j=cord
            for cand in [(i-1,j),(i+1,j),(i,j-1),(i,j+1)]:
                if cand in dic[ch]:
                    dic[ch].remove(cand)
                    flag=dfs(cand,indx+1)
                    dic[ch].add(cand)
                    if flag:return True
            return False
        

        lngth=len(word)
        for cord in list(dic[word[0]]):
            dic[word[0]].remove(cord)
            flag=dfs(cord,1)
            dic[word[0]].add(cord)
            if flag:return True
        return False

how to store the location that previously visited