Little C's Notepad

My idea is to record each inserted string , This means that only the current status is recorded , Discard the previous state directly

This solution is modified on the basis of the previous state , At the same time, the information of the previous state is not discarded . And I lost the information of the previous status , It takes time to find the last state .

Sometimes to save time , Requires a lot of space , Record some extra information , This additional information depends on the situation

#include<bits/stdc++.h>
using namespace std;

#define rep(i,l,r) for(int i=(l);i<=(r);i++)
#define per(i,l,r) for(int i=(l);i>=(r);i--)
#define ll long long
#define mset(s,t) memset(s,t,sizeof(t))
#define mcpy(s,t) memcpy(s,t,sizeof(t))
#define fi first
#define se second
#define pb push_back
#define all(x) (x).begin(),(x).end()
#define SZ(x) ((int)(x).size())
#define mp make_pair

typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;   
typedef vector<ll> Vll;               
typedef vector<pair<int, int> > vpii;
typedef vector<pair<ll, ll> > vpll;                        

const ll mod = 1e9 + 7;
//const ll mod = 998244353;
const double pi  = acos(-1.0);
inline ll ksc(ll x,ll y,ll mod)
{
    ll ans = 0;
    while (y) {
    	if (y & 1)
    		ans = (ans + x) %mod;
    	y >>= 1;
    	x = (x + x) %mod;
	}
	return ans;
}
inline ll qmi (ll a, ll b) {
	ll ans = 1;
	while (b) {
		if (b & 1) ans = ans * a;
		a = a * a;
		b >>= 1;
	}
	return ans;
}
inline int read () {
	int x = 0, f = 0;
	char ch = getchar();
	while (!isdigit(ch)) f |= (ch=='-'),ch= getchar();
	while (isdigit(ch)) x = x * 10 + ch - '0', ch = getchar();
	return f?-x:x;
}
template<typename T> void print(T x) {
	if (x < 0) putchar('-'), x = -x;
	if (x >= 10) print(x/10);
	putchar(x % 10 + '0');
}
inline ll sub (ll a, ll b) {
	return ((a - b ) %mod + mod) %mod;
}
inline ll add (ll a, ll b) {
	return (a + b) %mod;
}
// inline ll inv (ll a) {
	// return qmi(a, mod - 2);
// }


string s = "";
vector<string> a, b ;

int main () {
	// ios::sync_with_stdio(0),cin.tie(0), cout.tie(0);
    int t;
    while(cin >> t) {
    	int q;
    	string s = "";
    	stack<string> str;
    	str.push("");
    	while(t --) {
    		int x;
    		cin >> x;
    		if (x == 1) {
    			string temp;
    			cin >> temp;
    			str.push(str.top() + temp);
    		}
    		else if (x == 2) {
    			int k;
    			cin >> k;
    			string ss = str.top();
    			str.push(ss.substr(0, ss.size() - k));
    		}
    		else if (x == 3) {
    			int k;
    			cin >> k;
    			s = str.top();
    			cout <<  s[k - 1] << endl;
    		}
    		else {
    			str.pop();
    		}
    	}
    }
    
    return 0;
}