在计数时记得特殊判断0的长度为1:

int count(int x){
    if(x==0) return 1;//特判
    int res=0;
    if(x<0) res++;
    while(x!=0){
        x/=10;
        res++;
    }
    return res;
}

AC_code:

#include<iostream>
#include<sstream>
using namespace std;
int n,x,y;
char op;
string s;
int count(int x){
    if(x==0) return 1;
    int res=0;
    if(x<0) res++;
    while(x!=0){
        x/=10;
        res++;
    }
    return res;
}
void show(char op,int x,int y){
    if(op=='a') cout<<x<<"+"<<y<<"="<<x+y<<endl<<2+count(x)+count(y)+count(x+y)<<endl;
    else if(op=='b') cout<<x<<"-"<<y<<"="<<x-y<<endl<<2+count(x)+count(y)+count(x-y)<<endl;
    else cout<<x<<"*"<<y<<"="<<x*y<<endl<<2+count(x)+count(y)+count(x*y)<<endl;
}
int main(){
    cin>>n;
    getchar();
    while(n--){
        getline(cin,s);
        int flag=0;
        for(int i=0;i<s.size();i++) if(s[i]==' ') flag++;
        stringstream in(s);
        if(flag==1) in>>x>>y;
        else in>>op>>x>>y;
        show(op,x,y);
    }
    return 0;
}