任意长度的两个正整数相乘

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两个大数相乘：char* multiply(char*,char*)。给了两个字符串，每个都是代表了一个很长的10进制表示的数， 比如 String str1 = "23456789009877666555544444"； String str2 = "346587436598437594375943875943875"， 最后求出它们的乘积。 不用jdk的数学运算包BigInteger.或者类似的包。

 

#include <stdio.h>

#include <stdlib.h>

#include <string.h>



#define MULTIPLY_SIZE 200



int m_atoi(char c);

char *multiply(char *s, char *muldstr, char *mulerstr);

char *reverse_string(char *str);



//ascii to char

int m_atoi(char c){

    return ((c>='0')&&(c<='9'))? (c-'0') : 0;

}



char *multiply(char *s, char *muldstr, char *mulerstr){

    int muldlen = strlen(muldstr); 

    int    mulerlen = strlen(mulerstr);



    int count = 0;

    int i, j;

    for(i=muldlen-1; i>=0; i--, count++){

        char *ptrs = s + count;  

        int carrier = 0;

        int muld = m_atoi(muldstr[i]);

        for(j=mulerlen-1; j>=0; j--){

            int mul, temp, mod;



            temp = (m_atoi(*ptrs)+carrier);



            mul = m_atoi(mulerstr[j]);

            temp = temp + muld * mul;

            

            *ptrs = temp % 10 + '0';

            carrier = temp/10;



            ptrs++;

        }

        if(carrier != 0){

            *ptrs = carrier + '0';

        }

    }

    reverse_string(s);

    return s;

}



char *reverse_string(char *s){

    int len = strlen(s);

    char *first = s;

    char *last = s + len -1;

                

    while(last > first){

        char c = *first;

        *first = *last;

        *last = c;

        

        first++;

        last--;

    }

    return s;

}



int main(int argc, char *argv[]){

    /*

    char s1[200];

    char s2[200];

    char *s;



    scanf_s("%s", s1, sizeof(s1));

    scanf_s("%s", s2, sizeof(s2));

    */

    

    char s1[200] = "23456789009877666555544444";

    char s2[200] = "346587436598437594375943875943875";

    char *s;

    

    s = (char *)malloc(sizeof(char)*MULTIPLY_SIZE);

    memset(s, 0, MULTIPLY_SIZE);

    multiply(s, s1, s2);



    printf("%s\n", s);

    free(s);



    system("pause");

    return 0;

}

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