multiply-strings.py

# Time:  O(m * n)
# Space: O(m + n)

class Solution(object):
    def multiply(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        result = [0]*(len(num1)+len(num2))
        for i in reversed(xrange(len(num1))):
            for j in reversed(xrange(len(num2))):
                result[i+j+1] += int(num1[i])*int(num2[j])
                result[i+j] += result[i+j+1]//10
                result[i+j+1] %= 10
        for i in xrange(len(result)):
            if result[i]:
                break
        return "".join(map(lambda x: str(x), result[i:]))

# Time:  O(m * n)
# Space: O(m + n)
class Solution2(object):
    def multiply(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        num1, num2 = num1[::-1], num2[::-1]
        result = [0]*(len(num1)+len(num2))
        for i in xrange(len(num1)):
            for j in xrange(len(num2)):
                result[i+j] += int(num1[i])*int(num2[j])
                result[i+j+1] += result[i+j]//10
                result[i+j] %= 10
        for i in reversed(xrange(len(result))):
            if result[i]:
                break
        return "".join(map(str, result[i::-1]))

# Time:  O(m * n)
# Space: O(m + n)
# Using built-in bignum solution.
class Solution3(object):
    def multiply(self, num1, num2):
        """
        :type num1: str
        :type num2: str
        :rtype: str
        """
        return str(int(num1) * int(num2))