C++实现大数乘法算法代码

C++实现大数乘法算法代码

//大数乘法算法

#include<iostream>

#include<string>

#include<cstring>

using namespace std;

int main()

{

    string num1,num2;

    cin >> num1 >> num2;

    //cout << num1.size() << " " << num2.size() << endl;

    const char* n1;

    const char* n2;

    if (num1.size() < num2.size())

    {

        n1 = num2.c_str();

        n2 = num1.c_str();

    }

    else

    {

        n1 = num1.c_str();

        n2 = num2.c_str();

    }

    char* n = new char[strlen(n1)+strlen(n2)+1];

    for (unsigned int i = 0; i < strlen(n1)+strlen(n2); i++)

        n[i] = '0';

    n[strlen(n1)+strlen(n2)]='\0';

    //cout << strlen(n) << endl;

    int count = 0,flag = 0;

    for (int i = strlen(n1)-1; i >= 0; i--)

    {

        flag++;

        int x1 = n1[i]-'0';

        //cout << "n1["<< i << "]为：" << x1 << endl;

        char carry = '0';

        for (int j = strlen(n2)-1; j >= 0; j--)

        {

            int x2 = n2[j]-'0';

            //cout << "n2["<< j << "]为：" << x2 << endl;

            //cout << "当前位未改变前值为: " << n[count] << endl;

            int sum = x1*x2 + (carry-'0') + n[count]-'0';

            //cout << "sum is " << sum << endl;

            n[count++] = (sum % 10)+'0';

            carry = (sum / 10)+'0';

            //cout << "当前位的值为: " << n[count-1] << endl;

            //cout << "carry的值为：" << carry << endl;

        }

        if (carry != '0')

        {

            n[count] = carry;

            count = flag;

            //cout << "当前位的值为: " << n[count] << endl;

        }

        else

            count = flag;

    }

    for (int i = strlen(n)-1; i >= 0; i--)

    {

        if ((i == strlen(n)-1)&&(n[i] == '0'))

            continue;

        cout << n[i];

    }

    cout << endl;

    delete[]n;

    system("pause");

    return 0;

}

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