不用栈会很方便

#include <iostream>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>
#include <cmath>
#include <time.h>
#include <iomanip>
#include <cctype>
#include <string>
using namespace std;        //https://ac.nowcoder.com/acm/problem/16539
typedef long long ll;
const ll Mod = 10000;
stack<char> s1;   //符号栈
stack<ll> s2;   //数字栈
//遍历表达式，若是数字则入数字栈
//若是符号，比较当前符号与栈顶符号的优先级，若当前优先级不小于栈顶，则入符号栈
//若当前优先级小，则**将符号栈顶元素弹出，将数字栈顶两个元素弹出并进行对应计算后将结果重新压入数字栈**
//重复该操作直至栈顶元素优先级不大于当前符号，再将当前符号入栈（*保证符号栈的优先级递减性*）
//数字全部入栈后将两栈元素进行上述操作（** **）计算清空，最后数字栈中的元素即为答案

int trans(string x){        //转数字
    int ans=0;
    for(int j=0;j<x.length();j++){
        ans*=10;
        ans+=(int)(x[j]-'0');
    }
    return ans;
}
int cmp(char x){        //设置优先级
    if(x=='+'||x=='-') return 1;
    else if(x=='*'||x=='/') return 2;
    else return 0;
}
ll cal(char x,ll x1,ll x2){
    if(x=='+') return x1+x2;
    if(x=='-') return (x1-x2)%Mod;
    if(x=='*') return ((x1%Mod)*(x2%Mod))%Mod;
    if(x=='/') return x1/x2;
    return 0;
}
int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    string s;
    cin>>s;
    s+=".";
    string t="";
    for(int i=0;i<s.length();i++){
        if(isdigit(s[i]))
            t+=s[i];
        else if(s[i]=='.')
            s2.push(trans(t));
        else{
            s2.push(trans(t));
            t="";
            if(s1.empty()){
                s1.push(s[i]);
            }
            else{
                if(cmp(s[i])>=cmp(s1.top())){
                    s1.push(s[i]);
                }
                else{
                    while(cmp(s1.top())>cmp(s[i])&&!s1.empty()){
                        char tmp=s1.top();
                        s1.pop();
                        ll x1=s2.top();
                        s2.pop();
                        ll x2=s2.top();
                        s2.pop();
                        s2.push(cal(tmp,x2,x1));
                        if(s1.empty()) break;
                    }
                    s1.push(s[i]);
                }
            }
        }
    }

    while(!s1.empty()){
        char tmp=s1.top();
        s1.pop();
        ll x1=s2.top();
        s2.pop();
        ll x2=s2.top();
        s2.pop();
        s2.push(cal(tmp,x2,x1));
    }

    //cout<<s2.top()<<endl;
    cout<<s2.top()%Mod<<endl;
    return 0;
}