#include <vector> #include <iostream> using namespace std; const int mx = 1'000'000; const int lg_mx = 20; int curr; int len[1+mx]; char last[1+mx]; int letter_anc[1+mx][lg_mx]; int undo_anc[1+mx][lg_mx]; int lg2[1+mx]; void Init() { lg2[0] = -1; lg2[1] = 0; for(int i = 2; i <= mx; i++) lg2[i] = lg2[i/2] + 1; curr = 0; len[0] = -1; last[0] = '?'; for(int e = 0; e < lg_mx; e++) letter_anc[0][e] = undo_anc[0][e] = 0; } void TypeLetter(char L) { curr++; len[curr] = len[curr-1] + 1; last[curr] = L; letter_anc[curr][0] = curr-1; undo_anc[curr][0] = curr-1; for(int e = 1; e < max(lg2[curr], lg2[len[curr]])+2; e++) { letter_anc[curr][e] = letter_anc[ letter_anc[curr][e-1] ][e-1]; undo_anc[curr][e] = undo_anc[ undo_anc[curr][e-1] ][e-1]; } // cerr << curr << '\n'; } void UndoCommands(int U) { int prev = curr; for(int e = 0; e < lg2[curr]+2; e++) { if(U & (1 << e)) prev = undo_anc[prev][e]; } // cerr << "making copy of " << prev << '\n'; curr++; len[curr] = len[prev]; last[curr] = last[prev]; letter_anc[curr][0] = letter_anc[prev][0]; undo_anc[curr][0] = curr-1; for(int e = 1; e < max(lg2[curr], lg2[len[curr]])+2; e++) { letter_anc[curr][e] = letter_anc[ letter_anc[curr][e-1] ][e-1]; undo_anc[curr][e] = undo_anc[ undo_anc[curr][e-1] ][e-1]; } // cerr << curr << '\n'; } char GetLetter(int P) { int pos = curr; for(int e = max(lg2[curr], lg2[len[curr]])+2; e >= 0; e--) if(len[ letter_anc[pos][e] ] >= P) pos = letter_anc[pos][e]; return last[pos]; }