43.c

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

char*multiply(char*num1, char*num2) {
if (num1==NULL||num2==NULL) returnNULL;

intlen1=strlen(num1);
intlen2=strlen(num2);

int*prod= (int*)calloc(len1+len2+1, sizeof(int));
inti, j, k=0;
for (i=len1-1; i >= 0; i--) {
k=len1-1-i;
for (j=len2-1; j >= 0; j--) {
prod[k++] += (num1[i] -'0') * (num2[j] -'0');
 }
 }

k++; /* the last carry digit */

/* carry all */
intc=0;
for (i=0; i<k-1; i++) {
c=prod[i] / 10;
prod[i] =prod[i] % 10;
prod[i+1] +=c;
 }

/* remove lead zeros */
while (k>1&&prod[k-1] ==0) k--;

char*ans= (char*)calloc(k+1, sizeof(char));
/* reverse */
for (i=0; i<k; i++) {
ans[i] =prod[k-1-i] +'0';
 }
ans[k] ='\0';

free(prod);

returnans;
}

intmain() {
charnum1[] ="123456789";
charnum2[] ="987654321";

printf("%s\n", multiply(num1, num2));

return0;
}