[LeetCode 43]Multiply Strings

记录加入Datawhale第四天（中间出去了三天没有按时正是罪过呀），养成每天做题的好习惯

题目描述：

　　Given two non-negative integers num1 and num2 represented as strings, return the product of num1 and num2, also represented as a string.

题目要求不能使用库函数直接转换为整数，因此想到的是将结果保存到一个数组中在转化为字符串

Java代码：

 1 class Solution {
 2    public String multiply(String num1, String num2) {
 3        int[] ret = new int[num1.length() + num2.length()];
 4         int n = ret.length - 1;
 5         for (int i = num2.length() - 1; i >= 0; i--) {
 6             char c1 = num2.charAt(i);
 7             int index = n;
 8             for (int j = num1.length() - 1; j >= 0; j--) {
 9                 char c2 = num1.charAt(j);
10                 int tmp = (c1 - '0') * (c2 - '0');
11                 ret[index] += tmp % 10;
12                 ret[index - 1] += tmp / 10;
13                 index--;
14             }
15             n--;
16         }
17         StringBuilder str = new StringBuilder();
18         int i = ret.length - 1;
19         for (; i > 0; i--) {
20             int tmp = ret[i];
21             ret[i] = tmp % 10;
22             ret[i - 1] += tmp / 10;
23             str.append(ret[i]);
24         }
25         if(ret[i] > 0) str.append(ret[i]);
26         str.reverse();
27         int start = 0;
28         while(str.charAt(start) == '0' && start < str.length()-1){//去除前面的0
29             if(str.charAt(start) != '0') break;
30             str.deleteCharAt(start);
31         }
32         return str.toString();
33     }
34 }