【Leetcode】【Medium】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.

解题：

模拟乘法运算，可以完全按照模拟的思路，用num1的每一位乘num2，得到的数组按位保存在结果字符串中，并不断更新。

先把字符串反转，在逻辑上思路会更加清晰，当然不反转也可以。

 class Solution {
 public:
     string multiply(string num1, string num2) {
         reverse(num1.begin(), num1.end());
         reverse(num2.begin(), num2.end());
         int l1 = num1.size();
         int l2 = num2.size();
         string res(l1 + l2, '');
         int carry, d;

         for (int i = ; i < l1; ++i) {
             int n1 = num1[i] - '';
             carry = ;
             for (int j = ; j < l2; ++j) {
                 int n2 = num2[j] - '';
                 d = n1 * n2 + carry + (res[i+j] - '');
                 carry = d / ;
                 res[i+j] = '' + (d - carry * );
             }

             int idx = ;
             while (carry != ) {
                 d = (res[i+l2+idx] - '') + carry;
                 carry = d / ;
                 res[i+l2] = '' + (d - carry * );
                 idx++;
             }
         }

         while (!res.empty() && res.back() == '')
             res.pop_back();
         if (res.empty())
             return "";
         reverse(res.begin(), res.end());
         return res;
     }
 };