    def minDistance(self, word1: str, word2: str) -> int:
        n1,n2 = len(word1),len(word2)
        dp = [[0]*(n2+1) for _ in range(n1+1)]
        for i in range(n1+1):
            dp[i][0] = i
        for j in range(n2+1):
            dp[0][j] = j
        for i in range(1, n1+1):
            for j in range(1, n2+1):
                if word1[i-1] == word2[j-1]:
                    dp[i][j] = dp[i-1][j-1]
                else:
                    dp[i][j] = 1+ min(dp[i-1][j],dp[i][j-1],dp[i-1][j-1])
        return dp[-1][-1]