```
class Solution(object):
def minDistance(self, word1, word2):
m, n = map(len, (word1, word2))
dp = [range(n + 1)] + [[i] + [0] * n for i in xrange(1, m + 1)]
for i in xrange(1, m + 1):
for j in xrange(1, n + 1):
dp[i][j] = min(
dp[i][j - 1] + 1,
dp[i - 1][j] + 1,
dp[i - 1][j - 1] + (word1[i - 1] != word2[j - 1])
)
return dp[m][n]
```