#include <bits/stdc++.h>
using namespace std;
constexpr size_t MAX = 1 << 20, MAX_LOG = __lg(MAX);
struct node
{
char ch;
node* ptr[MAX_LOG];
} epsilon;
size_t length(node* curr)
{
if(curr == &epsilon)
return 0;
size_t r = 0;
for(size_t k = MAX_LOG; k --> 0; )
if(curr->ptr[k] != &epsilon)
curr = curr->ptr[k], r += 1 << k;
return r + 1;
}
vector<node*> history;
void Init()
{
history.reserve(MAX);
epsilon.ch = 0;
fill(begin(epsilon.ptr), end(epsilon.ptr), &epsilon);
history.push_back(&epsilon);
}
void TypeLetter(char c)
{
node* next = new node;
next->ch = c;
next->ptr[0] = history.back();
for(size_t k = 1; k < MAX_LOG; k++)
next->ptr[k] = (next->ptr[k-1])->ptr[k-1];
history.push_back(next);
}
void UndoCommands(int i)
{
history.push_back(history.rbegin()[i]);
}
char GetLetter(int i)
{
node* curr = history.back();
i = length(curr) - i - 1;
for(size_t k = MAX_LOG; k --> 0; )
if(i >> k & 1)
curr = curr->ptr[k];
return curr->ch;
}
