# 43. Multiply Strings 字符串相乘

## 刷题内容

• https://leetcode.com/problems/multiply-strings
• https://leetcode-cn.com/problems/multiply-strings

## 难度: Medium

1. m位的数字乘以n位的数字的结果最大为m+n位：
• 99999 < 1000100 = 100000，最多为3+2 = 5位数。
2. 先将字符串逆序便于从最低位开始计算。
class Solution(object):
def multiply(self, num1, num2):
"""
:type num1: str
:type num2: str
:rtype: str
"""
lookup = {"0":0,"1":1,"2":2,"3":3,"4":4,"5":5,"6":6,"7":7,"8":8,"9":9} # 节省查找时间，避免无休止使用ord函数来得到数字
if num1 == '0' or num2 == '0':
return '0'
num1, num2 = num1[::-1], num2[::-1]

tmp_res = [0 for i in range(len(num1)+len(num2))]
for i in range(len(num1)):
for j in range(len(num2)):
tmp_res[i+j] += lookup[num1[i]] * lookup[num2[j]]

res = [0 for i in range(len(num1)+len(num2))]
for i in range(len(num1)+len(num2)):
res[i] = tmp_res[i] % 10
if i < len(num1)+len(num2)-1:
tmp_res[i+1] += tmp_res[i]/10
return ''.join(str(i) for i in res[::-1]).lstrip('0')  # 去掉最终结果头部可能存在的‘0’


input: num1, num2 = '91', '91'
tmp_res = [1,18,81,0]
res = [1,8,2,8]



apachecn/AiLearning