HJ2 计算某字符出现次数（C++）- 简单、字符串、哈希

题目链接：https://www.nowcoder.com/practice/a35ce98431874e3a820dbe4b2d0508b1?tpId=37&tqId=21224&rp=1&ru=%2Fexam%2Foj%2Fta&qru=%2Fexam%2Foj%2Fta&sourceUrl=%2Fexam%2Foj%2Fta%3Fpage%3D1%26tpId%3D37%26type%3D37&difficulty=undefined&judgeStatus=undefined&tags=&title=
题目如下：

#include
using namespace std;
int main(){
    string str;
    char ch;
    
    getline(cin,str);//读入一整行数据到str中
    cin>>ch;
    
    int res=0;
    int flag=-1;
    if(isalpha(ch)>0) flag=1;//判断要查询的是否是字母
    for(auto e:str){
        if(flag==1&&(e==ch||abs(e-ch)==32)) res++;//abs(X):X的绝对值
        else if(flag==-1&&e==ch) res++;
    }
        
    cout<