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] == '' and len(ans) > 1:
del ans[0]
return ''.join(ans)