经典dp

def (str_a, str_b):
    def charAt(s, i):
        return s[i-1]

    dp = [[0]*(len(str_b)+1) for _ in range(len(str_a)+1)]

    for i in range(1, len(str_a)+1):
        dp[i][0] = i

    for i in range(1, len(str_b)+1):
        dp[0][i] = i

    for i in range(1, len(str_a)+1):
        for j in range(1, len(str_b)+1):
            if charAt(str_a, i) == charAt(str_b, j):
                dp[i][j] = dp[i-1][j-1]
            else:
                dp[i][j] = min(dp[i-1][j], dp[i][j-1], dp[i-1][j-1])+1

    return dp[len(str_a)][len(str_b)]