[leetcode] Multiply Strings

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.

 

 

分析：

模拟手算乘法即可。分两部分：字符串的乘法，字符串的加法。

 

例如：

          99

*         12

------------------

         198

+        99

------------------

         297

 

class Solution
{
public:
  string add(string num1, string num2)
  {
    string ret = "";
    int flag = 0, temp = 0;
    int i = num1.size() - 1, j = num2.size() - 1;

    for(; i >= 0 || j >= 0; --i, --j)
    {
      if(i >= 0 && j >= 0)
        temp = num1[i] - '0' + num2[j] - '0' + flag;
      else if(i >= 0 && j < 0)
        temp = num1[i] - '0' + flag;
      else
        temp = num2[j] - '0' + flag;

      flag = temp / 10;
      temp = temp % 10;
      ret.insert(ret.begin(), '0' + temp);
    }

    if(flag == 1)
      ret.insert(ret.begin(), '1');

    return ret;
  }
  
public:
  string multiply(string num1, string num2)
  {
    if(num1 == "0" || num2 == "0")
      return "0";

    string ret = "", str = "";
    int i = 0, j = 0, temp = 0, flag = 0;

    for(j = num2.size()-1; j>=0; --j)
    {
      str = "";
      flag = 0;
      for(i = num1.size()-1; i>=0; --i)
      {
        temp = (num1[i] - '0') * (num2[j] - '0') + flag;
        flag = temp / 10;
        temp = temp % 10;
        str.insert(str.begin(), temp + '0');
      }
      if(flag > 0)
        str.insert(str.begin(), flag + '0');
      
      for(int k = j; k < num2.size()-1; k++)
        str += "0";
      
      ret = add(str, ret);
    }

    return ret;
  }
};