#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;
}

为什么要用双栈

一个操作数栈，一个操作符栈。