leetcode 583 delete operation for two strings

class (object):
    def minDistance(self, w1, w2):
        """
        :type word1: str
        :type word2: str
        :rtype: int
        """
        if not w1: return len(w2)
        if not w2: return len(w1)
        m,n = len(w1), len(w2)
        dp = [[1]*n for _ in range(m)]
        for i in range(m):
            if w1[i] != w2[0]: dp[i][0] = 0
            else: break
        for j in range(n):
            if w1[0] != w2[j]: dp[0][j] = 0
            else: break

        for i in range(1,m):
            for j in range(1,n):
                if w1[i] == w2[j]:
                    dp[i][j] = dp[i-1][j-1] + 1
                else:
                    dp[i][j] = max(dp[i-1][j], dp[i][j-1])
        l = dp[-1][-1]
        return m - l + n - l

1307 / 1307 test cases passed.
diffculty: medium
Runtime: 180 ms