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Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

Example 1:

Input: num1 = "2", num2 = "3"Output: "6"

Example 2:

Input: num1 = "123", num2 = "456"Output: "56088"

计算大数相乘。

比较简单的思路就是直接根据每个数字所在位置进行计算。如下所示

class Solution:    def multiply(self, num1: str, num2: str) -> str:        res=0        for i,n1 in enumerate(num1[::-1]):            for j,n2 in enumerate(num2[::-1]):                res+=int(n1)*(10**i)*int(n2)*(10**j)        return str(res)

或者用dp算法，能快一些。计算dp[i+j]初的结果，然后dp[i+j+1]要进位，然后dp[i+j]再取余数（回归一位数）。

然后去除数字首位的0，返回字符串。

class Solution:    def multiply(self, num1: str, num2: str) -> str:        dp=[0]*(len(num1)+len(num2))                for i,n1 in enumerate(num1[::-1]):            for j,n2 in enumerate(num2[::-1]):                dp[i+j]+=int(n1)*int(n2)                dp[i+j+1]+=dp[i+j]//10                dp[i+j]%=10                while dp[-1]==0 and len(dp)>1:dp.pop()        return ''.join(str(a) for a in dp[::-1])

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