把 horse 改成 ros 要几步

每步可以插入、删除或替换一个字符。运行下面这段程序：

def edit(a, b):
    m = len(a)
    n = len(b)
    dp = [[0] * (n + 1) for _ in range(m + 1)]
    for i in range(m + 1):
        dp[i][0] = i
    for j in range(n + 1):
        dp[0][j] = j
    for i in range(1, m + 1):
        for j in range(1, n + 1):
            if a[i - 1] == b[j - 1]:
                dp[i][j] = dp[i - 1][j - 1]
            else:
                dp[i][j] = 1 + min(dp[i - 1][j - 1],
                                   dp[i - 1][j], dp[i][j - 1])
    return dp[m][n]

print(edit("horse", "ros"))