[leetcode]Multiply Strings @ Python

原题地址：https://oj.leetcode.com/problems/multiply-strings/

题意：

Given two numbers represented as strings, return multiplication of the numbers as a string.

Note: The numbers can be arbitrarily large and are non-negative.

解题思路：两个非负数字字符串的相乘。其实就是大数乘法。算法的关键是要先将两个字符串翻转过来，然后按位进行相乘，相乘后的数不要着急进位，而是存储在一个数组里面，然后将数组中的数对10进行求余（%），就是这一位的数，然后除以10，即/10，就是进位的数。注意最后要将相乘后的字符串前面的0去掉。

代码：

class Solution:
    # @param num1, a string
    # @param num2, a string
    # @return a string
    def multiply(self, num1, num2):
        num1 = num1[::-1]; num2 = num2[::-1]
        arr = [0 for i in range(len(num1)+len(num2))]
        for i in range(len(num1)):
            for j in range(len(num2)):
                arr[i+j] += int(num1[i]) * int(num2[j])
        ans = []
        for i in range(len(arr)):
            digit = arr[i] % 10
            carry = arr[i] / 10
            if i < len(arr)-1:
                arr[i+1] += carry
            ans.insert(0, str(digit))
        while ans[0] == '0' and len(ans) > 1:
            del ans[0]
        return ''.join(ans)