大数相乘

#include <stdio.h>
#include <string.h>

#define SIZE 1024

void multiply(const char *num1, const char *num2, char *result)
{
    if(num1 == NULL || num2 == NULL)
        return ;

    //判断0
    if(num1[0] == '0' || num2[0] == '0')
    {
        result[0] = '0';
        return;
    }

    int isNegative = 1;
    int beg1 = 0, beg2 = 0, end1 = strlen(num1) - 1, end2 = strlen(num2) - 1;
    int i, j, k = 0;
    int carry = 0, record = 0;
    int mult1 = 0, mult2 = 0, mult = 0;

    //判断符号
    if(num1[0] == '-' && num2[0] != '-')
    {
        isNegative = -1;
        beg1++;
    }
    else if(num1[0] != '-' && num2[0] == '-')
    {
        isNegative = -1;
        beg2++;
    }
    else if(num1[0] == '-' && num2[0] == '-')
    {
        isNegative = 1;
        beg1++;
        beg2++;
    }
    else
        isNegative = 1;

    //两大数相乘
    for(i = end1; i >= beg1; i--)
    {
        if(num1[i] >= '0' && num1[i] <= '9')
            mult1 = num1[i] - '0';
        k = record++;
        carry = 0;
        for(j = end2; j >= beg2; j--)
        {
            if(num2[j] >= '0' && num2[j] <= '9')
                mult2 = num2[j] - '0';
            if(result[k] == 0)
                result[k] += '0';
            mult = mult1 * mult2 + carry + result[k] - '0';
            result[k++] = mult % 10 + '0';
            carry = mult / 10;
        }
        if(carry > 0)
            result[k++] = carry + '0';
    }

    //如果是负数
    if(isNegative == -1)
    {
        result[k++] = '-';
    }

    //字符串逆序
    i = 0;
    j = strlen(result) - 1;
    for(; i <= j; i++, j--)
    {
        char c = result[i];
        result[i] = result[j];
        result[j] = c;
    }
}

int main()
{
    char num1[SIZE] = "-990";
    char num2[SIZE] = "102030409";
    char result[SIZE];
    memset(result, 0, sizeof(result));
    multiply(num1, num2, result);
    printf("%s\n", result);
}