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注意：
要去掉前面的0再輸出。
 #include <stdio.h>
#include <string.h>
  struct node  {
 char arr[128];
int len;
 };
__inline int max(int a, int b)
{
return a > b ? a : b;
}
__inline void add(struct node *a, struct node *b)
{
int i, val;
  for (val = i = 0; i < max(a->len, b->len); i++) {
if (i < a->len)
val += a->arr[i];
if (i < b->len)
val += b->arr[i];
a->arr[i] = val % 10;
val /= 10;
 }
if (val)
a->arr[i++] = val;
a->len = i;
}
__inline void mul_single(struct node *a, int val, int pad, struct node *c)
{
int i, r;
for (i = 0; i < pad; i++)
c->arr[i] = 0;
c->len = pad;
for (r = i = 0; i < a->len; i++) {
r += a->arr[i] * val;
c->arr[c->len++] = r % 10;
r /= 10;
}
if (r)
c->arr[c->len++] = r;
}
__inline void mul(struct node *a, struct node *b, struct node *c)
{
struct node t;
int i;
c->len = 0;
for (i = 0; i < a->len; i++) {
mul_single(b, a->arr[i], i, &t);
add(c, &t);
}
}
__inline void input(struct node *t)
{
char str[sizeof(t->arr)];
int i, len;
scanf("%s", str);
len = strlen(str);
t->len = 0;
for (i = len - 1; i >= 0; i--)
t->arr[t->len++] = str[i] - '0';
}
__inline void output(struct node *t)
{
int i;
for (i = t->len - 1; i >= 0 && !t->arr[i]; i--);
if (i < 0) {
printf("0\n");
return ;
}
for ( ; i >= 0; i--)
putc(t->arr[i] + '0', stdout);
putc('\n', stdout);
}
int main()
{
struct node a, b, c;
freopen("e:\\test\\in.txt", "r", stdin);
input(&a);
input(&b);
mul(&a, &b, &c);
output(&c);
return 0;
}
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