文章作者：Tyan
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1. Description

Multiply Strings

2. Solution

解析：Version 1，直接按照學(xué)習(xí)的乘法計(jì)算方式來求解，先計(jì)算第二個(gè)數(shù)的每一位（從個(gè)位開始）乘以第一個(gè)數(shù)，再將得出的結(jié)果加到一起。Version 2采用另一種方式實(shí)現(xiàn)。

• Version 1

class Solution:
    def multiply(self, num1: str, num2: str) -> str:
        if num1 == '0' or num2 == '0':
            return '0'
        result = []
        carry = 0
        num1 = list(map(int, num1))
        num2 = list(map(int, num2))
        for i in range(len(num2) -1, -1, -1):
            if num2[i] == 0:
                continue
            temp = self.mul(num1, num2[i], len(num2) - i - 1)
            result = self.add(result, temp)
        p = [str(num) for num in result]
        return ''.join(p)[::-1]


    def mul(self, num1, b, index):
        result = []
        carry = 0
        for a in reversed(num1):
            product = a * b + carry
            result.append(product % 10)
            carry = product // 10
        if carry > 0:
            result.append(carry)
        return [0] * index + result


    def add(self, a, b):
        carry = 0
        for i in range(len(b)):
            if i < len(a):
                x = a[i]
            else:
                x = 0
            y = b[i]
            sum = x + y + carry
            b[i] = sum % 10
            carry = sum // 10
        if carry > 0:
            b.append(carry)
        return b

• Version 2

class Solution:
    def multiply(self, num1: str, num2: str) -> str:
        if num1 == '0' or num2 == '0':
            return '0'
        num1 = list(map(int, num1))
        num2 = list(map(int, num2))
        m = len(num1)
        n = len(num2)
        result = [0] * (m + n)
        for i in range(m-1, -1, -1):
            for j in range(n-1, -1, -1):
                product = num1[i] * num2[j]
                sum = product + result[i+j+1]
                result[i+j+1] = sum % 10
                result[i+j] += sum // 10
        result = list(map(str, result))
        result = ''.join(result).lstrip('0')
        return result

Reference

1. https://leetcode.com/problems/multiply-strings//