#include <stdio.h>
#include <assert.h>

void BigNumMultiply(const char *str1, const char *str2, char *product)
{
    assert(str1 != NULL && str2 != NULL && product != NULL);

    int i, j;
    int len1 = (int)strlen(str1);
    int len2 = (int)strlen(str2);
    int *dest = (int*)malloc(sizeof(int)*(len1+len2+1));

    for (i=0; i<len1+len2+1; i++) { dest[i] = 0; }

    for (i=0; i<len1; i++)
    {
        for (j=0; j<len2; j++)
        {
            dest[i+j+1] += (str1[i]-'0')*(str2[j]-'0');
        }
    }

    for (i=len1+len2-1; i>=0; i--)
    {
        // 当i=0时dest[0]=0,这个条件不成立,所以不用担心dest[-1]
        if (dest[i] >= 10)
        {
            dest[i-1] += dest[i]/10;
            dest[i] %= 10;
        }
        product[i] = dest[i]+'0';
    }

    if (product[0] == '0')
    {
        i = 1;
        while (product[i] != '\0')
        {
            product[i-1] = product[i];
            i++;
        }
        product[i-1] = '\0';
    }

    free(dest);
    return;
}

int main(void)
{
    char product[50] = {0};

    BigNumMultiply("234324", "54651", product);
    printf("%s\n", product);

    getchar();
    return 0;
}

测试:


    string multiply(string num1, string num2) {
        std::string result;
        std::vector<int> data;
        int total = num1.size() + num2.size() + 1;
        data.resize(total, 0);
        for (std::size_t i=0; i<num1.size(); i++) {
            for (std::size_t j=0; j<num2.size(); j++) {
                data[i+j+1] += (num1[i]-'0') * (num2[j]-'0');
            }
        }
        for (int i=num1.size()+num2.size()-1; i>=0; i--) {
            if (data[i] >= 10) {
                data[i-1] += data[i] / 10;
                data[i] %= 10;
            }
            result.insert(result.begin(), data[i] + '0');
        }
        while (result.size() > 1 && result[0] == '0') {
            result = result.erase(0, 1);
        }
        return result;
    }

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