Solution to Leetcode 79 Word Search

soln_lc79.py

class Solution:
    def exist(self, board: List[List[str]], word: str) -> bool:
        ROWS, COLS = len(board), len(board[0])
        path = set()

        def dfs(r, c, i):
            if i == len(word):
                return True 
            if (r<0 or c<0 or r>= ROWS or c>= COLS
            or board[r][c] != word[i] or
            (r,c) in path
            ) : return False 
            # if any of the above conditions is met, 
            # it won't execute the following lines
            path.add((r,c))
            res = (
                dfs(r-1, c, i+1) or 
                dfs(r+1, c, i+1) or
                dfs(r, c-1, i+1) or
                dfs(r, c+1, i+1) 
            )
            path.remove((r,c))
            return res 
        
        for r in range(ROWS):
            for c in range(COLS):
                if dfs(r, c, 0): return True 
        
        return False