LeetCode Multiply Strings

LeetCode Multiply Strings
LeetCode-solving Multiply Strings

Original question

Multiply two numbers represented by strings and return the string results.

Note:

The given number is a non-negative integer and can be infinite.

Example:

Input: num1 = "123", num2 = "20"
Output: "2460"

Solutions

According to the formula of multiplication, the multiplication operation is actually to perform the multiplication operation on each bit first, and then perform the addition operation on all results. First, specify that the number of m-bit multiplied by the number of n-bit is mostly m + n-bit (when all are 9 ). The numbers equal to 0 are aligned at the end when they are added. For example, 123 × 456, we can see that 1 × 6, 2 × 5, and 3 × 4 are aligned at the end of addition, we can add these numbers in the first round of multiplication, and the following zeros are represented by positions in the list. Use a loop to carry out the carry addition, and finally remove the excess 0 at the beginning. For detailed steps, see the following example:

123*456100        400 20         50  3          6[3*6, 2*6+3*5, 1*6+2*5+3*4, 2*4+1*5, 1*4, 0][18, 27, 28, 13, 4, 0][8, 27+1, 28, 13, 4, 0][8, 8, 28+2, 13, 4, 0][8, 8, 0, 13+3, 4, 0][8, 8, 0, 6, 5, 0]"880650"-->"056088""56088"

AC Source Code

class Solution(object):    def multiply(self, num1, num2):        """        :type num1: str        :type num2: str        :rtype: str        """        num1 = num1[::-1]        num2 = num2[::-1]        length1 = len(num1)        length2 = len(num2)        temp = [0 for __ in range(length1 + length2)]        # Do multiply        for i in range(length1):            for j in range(length2):                temp[i + j] += int(num1[i]) * int(num2[j])        carry = 0        digits = []        # Do plus        for num in temp:            s = carry + num            carry = s // 10            digits.append(str(s % 10))        result = "".join(digits)[::-1]        # Remove the surplus zero        sub_index = 0        for i in range(length1 + length2 - 1):            if result[i] == "0":                sub_index += 1            else:                break        result = result[sub_index:]        return resultif __name__ == "__main__":    assert Solution().multiply("120", "20000") == 2400000    assert Solution().multiply("0", "3421") == 0

Please check out my Github for relevant source code.