不要用双栈，直接把乘号连接的数看成一个整体。
标程如下：
pascal
var
i,j,k,num,ans,x,num2:longint;
s:ansistring;
begin
readln(s);
k:=1;
ans:=0;
while k<=length(s) do
begin
num:=0;
while (s[k]>='0')and(s[k]<='9') do
begin
num:=(num*10+ord(s[k])-48)mod 10000;
inc(k);
end;
while s[k]='*' do
begin
num2:=0;
inc(k);
while (s[k]>='0')and(s[k]<='9') do
begin
num2:=(num2*10+ord(s[k])-48)mod 10000;
inc(k);
end;
num:=(num*num2)mod 10000;
end;
ans:=(ans+num)mod 10000;
inc(k);
end;
writeln(ans);
end.


#include<cstdio>
#include<cstring>
#define mod 10000
#define maxn (1000000 + 10)
#define f(x,y) for( register int x = 1;x <= y;x ++)
using namespace std;
char c[maxn];
int st[maxn];
int pos = 0;
#define top st[pos]
#define pop1() pos--
int main() {
    scanf("%s",c + 1);
    int len = strlen(c + 1);
    int temp = 0;
    f(i,len+1) { 
        if (c[i] >= '0' && c[i] <= '9') temp = (temp * 10 + c[i] - '0') % mod;
        else {
            if (top == -2) {
                pop1();
                int t = (top * temp) % mod;
                st[pos] = t;
            }else st[++ pos] = temp;
            if(c[i] == '*') st[++ pos] = -2;
            temp = 0;
        }
    }
    int ans = 0;
    pos ++;
    while(pos --) ans = (ans + st[pos]) % mod;
    printf ("%d",ans);
    return 0;
}

#include<cstdio>
#include<cstring>
#define mod 10000
#define maxn (1000000 + 10)
#define f(x,y) for( register int x = 1;x <= y;x ++)
using namespace std;
char c[maxn];
int st[maxn];
int pos = 0;
#define top st[pos]
#define pop1() pos--
int main() {
scanf("%s",c + 1);
int len = strlen(c + 1);
int temp = 0;
f(i,len+1) {
if (c[i] >= '0' && c[i] <= '9') temp = (temp * 10 + c[i] - '0') % mod;
else {
if (top == -2) {
pop1();
int t = (top * temp) % mod;
st[pos] = t;
}else st[++ pos] = temp;
if(c[i] == '*') st[++ pos] = -2;
temp = 0;
}
}
int ans = 0;
pos ++;
while(pos --) ans = (ans + st[pos]) % mod;
printf ("%d",ans);
return 0;
}