leetcode72.Edit Distance

Given two words word1 and word2, find the minimum number of steps required to convert word1 to word2. (each operation is counted as 1 step.)
You have the following 3 operations permitted on a word:
a) Insert a character
b) Delete a character
c) Replace a character

  w o r d 2

w [ 0 1 2 3 4 ]
o [ 1
r [ 2
d [ 3
1 [ 4
ret[0][j]=j
ret[i][0]=i
when A[i]==B[j]:
ret[i][j]=ret[i-1][j-1]
else:
ret[i][j]=min(ret[i-1][j-1],ret[i][j-1],ret[i-1][j])+1

class Solution(object):    def minDistance(self, word1, word2):        """        :type word1: str        :type word2: str        :rtype: int        """        n1=len(word1);n2=len(word2)        tmp=[0 for i in range(n2+1)]        ret=[tmp[:] for i in range(n1+1)]        for i in range(n1+1):            ret[i][0]=i        for j in range(n2+1):            ret[0][j]=j        for i in range(n1):            for j in range(n2):                if word1[i]==word2[j]:                    ret[i+1][j+1]=ret[i][j]                else:                    ret[i+1][j+1]=min(ret[i][j],ret[i][j+1],ret[i+1][j])+1        return ret[n1][n2]