[Leetcode] 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.

 

大数乘法，注意前置0的处理。

 

 1 class Solution {

 2 public:

 3     string multiply(string num1, string num2) {

 4         string num3;

 5         num3.resize(num1.size() + num2.size());

 6         reverse(num1.begin(), num1.end());

 7         reverse(num2.begin(), num2.end());

 8         int carry = 0;

 9         int val;

10         int i, j, k;

11         for (i = 0; i < num3.size(); ++i) {

12             num3[i] = '0';

13         }

14         for (i = 0; i < num1.size(); ++i) {

15             carry = 0;

16             for (j = 0; j < num2.size(); ++j) {

17                 val = (num3[i+j] - '0') + (num1[i] - '0') * (num2[j] - '0') + carry;

18                 carry = val / 10;

19                 val %= 10;

20                 num3[i+j] = '0' + val;

21             }

22             num3[i + j] = '0' + carry;

23         }

24         int prezero = 0;

25         for (i = num3.size() - 1; i > 0; --i) {

26             if (num3[i] == '0') 

27                 prezero++;

28             else 

29                 break;

30         }

31         num3.resize(num3.size() - prezero);

32         reverse(num3.begin(), num3.end());

33         return num3;

34     }

35 };