字符串函数会用即可

#include <bits/stdc++.h>
using namespace std;
string s,s1;
char c,x;
int main(){
	getline(cin,s);
	cin>>c;
	if (c=='D'){
		cin>>x;
		int now=s.find(x);
		if (now==-1){cout<<"Not exist";return 0;}
		s.erase(now,1);
		cout<<s;return 0;
	}else if (c=='I'){
		cin>>x>>s1;
		int now1=s.rfind(x);
		if (now1==-1){cout<<"Not exist";return 0;}
		s.insert(now1,s1);
		cout<<s;return 0;
	}else{
		cin>>x>>s1;
		bool flag=false;
		for (int i=0;i<s.size();i++){
			if (s[i]==x){s.replace(i,1,s1);flag=true;}
		}
		if (!flag){cout<<"Not exist";return 0;}
		cout<<s;return 0;
	}
	return 0;
}