[Leetcode]Multiply Strings

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.

大数乘法~依旧是从低位到高位进行运算，可以先把num1, num2反转以方便进行运算~注意最后要去掉结果数组中高位的0，并且对结果数组进行反转~

class Solution:
    # @param num1, a string
    # @param num2, a string
    # @return a string
    def multiply(self, num1, num2):
        if num1 is None or num2 is None or len(num1) == 0 or len(num2) == 0:
            return ""
        if num1[0] == '0' or num2[0] == '0': return '0'
        num1 = [ord(i) - ord('0') for i in num1][::-1]
        num2 = [ord(i) - ord('0') for i in num2][::-1]
        total = [0] * (len(num1) + len(num2))
        for i in xrange(len(num1)):
            for j in xrange(len(num2)):
                total[i + j] += num1[i] * num2[j]
        carry, res = 0, []
        for sum in total:
            sum += carry
            carry = sum / 10
            res.append(str(sum % 10))
        while len(res) > 1 and res[-1] == '0':
            res.pop()
        return ''.join(res[::-1])