Preduvjeti: Grundy brojevi/brojevi i mex
Već smo vidjeli u setu 2 (https://www.geeksforgeeks.org/dsa/combinatorial-game-theory-set-2-game-name-nim/) da možemo pronaći tko će pobjeđivati u igri NIM-a bez da zapravo igra igru.
Pretpostavimo da malo promijenimo klasičnu NIM igru. Ovaj put svaki igrač može ukloniti samo 1 2 ili 3 kamenja (a ne bilo koji broj kamenja kao u klasičnoj igri NIM -a). Možemo li predvidjeti tko će pobijediti?
Da, možemo predvidjeti pobjednika pomoću Sprague-Grundy teorema.
Što je Sprague-Grundy teorem?
Pretpostavimo da postoji složena igra (više od jedne pod-igre) sačinjena od N pod-igara i dva igrača A i B. Tada Sprague-Grundy teorem kaže da ako i A i B igraju optimalno (tj. Oni ne čine nikakve pogreške), tada je igrač koji započinje prvo zajamčeno da je XOR nebrojena u svakoj igri na početku. U suprotnom, ako XOR procjeni na nulu, igrač A će izgubiti definitivno bez obzira na sve.
Kako nanijeti teorem Sprague Grundy?
U bilo kojem možemo primijeniti teorem Sprague-Grundy nepristrana igra i riješiti ga. Osnovni koraci su navedeni na sljedeći način:
- Razbiti kompozitnu igru u pod-igre.
- Zatim za svaku pod-igru izračunajte grund broj na tom položaju.
- Zatim izračunajte XOR svih izračunatih grundnih brojeva.
- Ako je vrijednost XOR-a nije nula, tada će igrač koji će napraviti skretanje (prvi igrač) pobijediti drugi, koji mu je suđeno da izgubi bez obzira na sve.
Primjer igra: Igra započinje s 3 gomile koje imaju 3 4 i 5 kamenja, a igrač za pomicanje može uzeti bilo koji pozitivan broj kamenja do 3 samo iz bilo koje od gomila [pod uvjetom da gomila ima toliko kamenja]. Posljednji igrač koji se kreće pobjeđuje. Koji igrač pobjeđuje u igri pretpostavljajući da oba igrača igraju optimalno?
Kako znati tko će pobijediti primjenom Sprague-grund teorema?
Kao što vidimo da se ova igra sastoji od nekoliko pod-igara.
Prvi korak: Pod-igre se mogu uzeti u obzir kao svaka hrpa.
Drugi korak: Iz donje tablice vidimo da
Grundy(3) = 3 Grundy(4) = 0 Grundy(5) = 1
Već smo vidjeli kako izračunati grund brojeve ove igre u prethodni članak.
Treći korak: XOR od 3 0 1 = 2
Četvrti korak: Budući da je XOR ne-nulti broj, pa možemo reći da će prvi igrač pobijediti.
Ispod je program koji implementira iznad 4 koraka.
C++/* Game Description- 'A game is played between two players and there are N piles of stones such that each pile has certain number of stones. On his/her turn a player selects a pile and can take any non-zero number of stones upto 3 (i.e- 123) The player who cannot move is considered to lose the game (i.e. one who take the last stone is the winner). Can you find which player wins the game if both players play optimally (they don't make any mistake)? ' A Dynamic Programming approach to calculate Grundy Number and Mex and find the Winner using Sprague - Grundy Theorem. */ #include
import java.util.*; /* Game Description- 'A game is played between two players and there are N piles of stones such that each pile has certain number of stones. On his/her turn a player selects a pile and can take any non-zero number of stones upto 3 (i.e- 123) The player who cannot move is considered to lose the game (i.e. one who take the last stone is the winner). Can you find which player wins the game if both players play optimally (they don't make any mistake)? ' A Dynamic Programming approach to calculate Grundy Number and Mex and find the Winner using Sprague - Grundy Theorem. */ class GFG { /* piles[] -> Array having the initial count of stones/coins in each piles before the game has started. n -> Number of piles Grundy[] -> Array having the Grundy Number corresponding to the initial position of each piles in the game The piles[] and Grundy[] are having 0-based indexing*/ static int PLAYER1 = 1; static int PLAYER2 = 2; // A Function to calculate Mex of all the values in that set static int calculateMex(HashSet<Integer> Set) { int Mex = 0; while (Set.contains(Mex)) Mex++; return (Mex); } // A function to Compute Grundy Number of 'n' static int calculateGrundy(int n int Grundy[]) { Grundy[0] = 0; Grundy[1] = 1; Grundy[2] = 2; Grundy[3] = 3; if (Grundy[n] != -1) return (Grundy[n]); // A Hash Table HashSet<Integer> Set = new HashSet<Integer>(); for (int i = 1; i <= 3; i++) Set.add(calculateGrundy (n - i Grundy)); // Store the result Grundy[n] = calculateMex (Set); return (Grundy[n]); } // A function to declare the winner of the game static void declareWinner(int whoseTurn int piles[] int Grundy[] int n) { int xorValue = Grundy[piles[0]]; for (int i = 1; i <= n - 1; i++) xorValue = xorValue ^ Grundy[piles[i]]; if (xorValue != 0) { if (whoseTurn == PLAYER1) System.out.printf('Player 1 will winn'); else System.out.printf('Player 2 will winn'); } else { if (whoseTurn == PLAYER1) System.out.printf('Player 2 will winn'); else System.out.printf('Player 1 will winn'); } return; } // Driver code public static void main(String[] args) { // Test Case 1 int piles[] = {3 4 5}; int n = piles.length; // Find the maximum element int maximum = Arrays.stream(piles).max().getAsInt(); // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided int Grundy[] = new int[maximum + 1]; Arrays.fill(Grundy -1); // Calculate Grundy Value of piles[i] and store it for (int i = 0; i <= n - 1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER1 piles Grundy n); /* Test Case 2 int piles[] = {3 8 2}; int n = sizeof(piles)/sizeof(piles[0]); int maximum = *max_element (piles piles + n); // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided int Grundy [maximum + 1]; memset(Grundy -1 sizeof (Grundy)); // Calculate Grundy Value of piles[i] and store it for (int i=0; i<=n-1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER2 piles Grundy n); */ } } // This code is contributed by PrinciRaj1992
''' Game Description- 'A game is played between two players and there are N piles of stones such that each pile has certain number of stones. On his/her turn a player selects a pile and can take any non-zero number of stones upto 3 (i.e- 123) The player who cannot move is considered to lose the game (i.e. one who take the last stone is the winner). Can you find which player wins the game if both players play optimally (they don't make any mistake)? ' A Dynamic Programming approach to calculate Grundy Number and Mex and find the Winner using Sprague - Grundy Theorem. piles[] -> Array having the initial count of stones/coins in each piles before the game has started. n -> Number of piles Grundy[] -> Array having the Grundy Number corresponding to the initial position of each piles in the game The piles[] and Grundy[] are having 0-based indexing''' PLAYER1 = 1 PLAYER2 = 2 # A Function to calculate Mex of all # the values in that set def calculateMex(Set): Mex = 0; while (Mex in Set): Mex += 1 return (Mex) # A function to Compute Grundy Number of 'n' def calculateGrundy(n Grundy): Grundy[0] = 0 Grundy[1] = 1 Grundy[2] = 2 Grundy[3] = 3 if (Grundy[n] != -1): return (Grundy[n]) # A Hash Table Set = set() for i in range(1 4): Set.add(calculateGrundy(n - i Grundy)) # Store the result Grundy[n] = calculateMex(Set) return (Grundy[n]) # A function to declare the winner of the game def declareWinner(whoseTurn piles Grundy n): xorValue = Grundy[piles[0]]; for i in range(1 n): xorValue = (xorValue ^ Grundy[piles[i]]) if (xorValue != 0): if (whoseTurn == PLAYER1): print('Player 1 will winn'); else: print('Player 2 will winn'); else: if (whoseTurn == PLAYER1): print('Player 2 will winn'); else: print('Player 1 will winn'); # Driver code if __name__=='__main__': # Test Case 1 piles = [ 3 4 5 ] n = len(piles) # Find the maximum element maximum = max(piles) # An array to cache the sub-problems so that # re-computation of same sub-problems is avoided Grundy = [-1 for i in range(maximum + 1)]; # Calculate Grundy Value of piles[i] and store it for i in range(n): calculateGrundy(piles[i] Grundy); declareWinner(PLAYER1 piles Grundy n); ''' Test Case 2 int piles[] = {3 8 2}; int n = sizeof(piles)/sizeof(piles[0]); int maximum = *max_element (piles piles + n); // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided int Grundy [maximum + 1]; memset(Grundy -1 sizeof (Grundy)); // Calculate Grundy Value of piles[i] and store it for (int i=0; i<=n-1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER2 piles Grundy n); ''' # This code is contributed by rutvik_56
using System; using System.Linq; using System.Collections.Generic; /* Game Description- 'A game is played between two players and there are N piles of stones such that each pile has certain number of stones. On his/her turn a player selects a pile and can take any non-zero number of stones upto 3 (i.e- 123) The player who cannot move is considered to lose the game (i.e. one who take the last stone is the winner). Can you find which player wins the game if both players play optimally (they don't make any mistake)? ' A Dynamic Programming approach to calculate Grundy Number and Mex and find the Winner using Sprague - Grundy Theorem. */ class GFG { /* piles[] -> Array having the initial count of stones/coins in each piles before the game has started. n -> Number of piles Grundy[] -> Array having the Grundy Number corresponding to the initial position of each piles in the game The piles[] and Grundy[] are having 0-based indexing*/ static int PLAYER1 = 1; //static int PLAYER2 = 2; // A Function to calculate Mex of all the values in that set static int calculateMex(HashSet<int> Set) { int Mex = 0; while (Set.Contains(Mex)) Mex++; return (Mex); } // A function to Compute Grundy Number of 'n' static int calculateGrundy(int n int []Grundy) { Grundy[0] = 0; Grundy[1] = 1; Grundy[2] = 2; Grundy[3] = 3; if (Grundy[n] != -1) return (Grundy[n]); // A Hash Table HashSet<int> Set = new HashSet<int>(); for (int i = 1; i <= 3; i++) Set.Add(calculateGrundy (n - i Grundy)); // Store the result Grundy[n] = calculateMex (Set); return (Grundy[n]); } // A function to declare the winner of the game static void declareWinner(int whoseTurn int []piles int []Grundy int n) { int xorValue = Grundy[piles[0]]; for (int i = 1; i <= n - 1; i++) xorValue = xorValue ^ Grundy[piles[i]]; if (xorValue != 0) { if (whoseTurn == PLAYER1) Console.Write('Player 1 will winn'); else Console.Write('Player 2 will winn'); } else { if (whoseTurn == PLAYER1) Console.Write('Player 2 will winn'); else Console.Write('Player 1 will winn'); } return; } // Driver code static void Main() { // Test Case 1 int []piles = {3 4 5}; int n = piles.Length; // Find the maximum element int maximum = piles.Max(); // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided int []Grundy = new int[maximum + 1]; Array.Fill(Grundy -1); // Calculate Grundy Value of piles[i] and store it for (int i = 0; i <= n - 1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER1 piles Grundy n); /* Test Case 2 int piles[] = {3 8 2}; int n = sizeof(piles)/sizeof(piles[0]); int maximum = *max_element (piles piles + n); // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided int Grundy [maximum + 1]; memset(Grundy -1 sizeof (Grundy)); // Calculate Grundy Value of piles[i] and store it for (int i=0; i<=n-1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER2 piles Grundy n); */ } } // This code is contributed by mits
<script> /* Game Description- 'A game is played between two players and there are N piles of stones such that each pile has certain number of stones. On his/her turn a player selects a pile and can take any non-zero number of stones upto 3 (i.e- 123) The player who cannot move is considered to lose the game (i.e. one who take the last stone is the winner). Can you find which player wins the game if both players play optimally (they don't make any mistake)? ' A Dynamic Programming approach to calculate Grundy Number and Mex and find the Winner using Sprague - Grundy Theorem. */ /* piles[] -> Array having the initial count of stones/coins in each piles before the game has started. n -> Number of piles Grundy[] -> Array having the Grundy Number corresponding to the initial position of each piles in the game The piles[] and Grundy[] are having 0-based indexing*/ let PLAYER1 = 1; let PLAYER2 = 2; // A Function to calculate Mex of all the values in that set function calculateMex(Set) { let Mex = 0; while (Set.has(Mex)) Mex++; return (Mex); } // A function to Compute Grundy Number of 'n' function calculateGrundy(nGrundy) { Grundy[0] = 0; Grundy[1] = 1; Grundy[2] = 2; Grundy[3] = 3; if (Grundy[n] != -1) return (Grundy[n]); // A Hash Table let Set = new Set(); for (let i = 1; i <= 3; i++) Set.add(calculateGrundy (n - i Grundy)); // Store the result Grundy[n] = calculateMex (Set); return (Grundy[n]); } // A function to declare the winner of the game function declareWinner(whoseTurnpilesGrundyn) { let xorValue = Grundy[piles[0]]; for (let i = 1; i <= n - 1; i++) xorValue = xorValue ^ Grundy[piles[i]]; if (xorValue != 0) { if (whoseTurn == PLAYER1) document.write('Player 1 will win
'); else document.write('Player 2 will win
'); } else { if (whoseTurn == PLAYER1) document.write('Player 2 will win
'); else document.write('Player 1 will win
'); } return; } // Driver code // Test Case 1 let piles = [3 4 5]; let n = piles.length; // Find the maximum element let maximum = Math.max(...piles) // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided let Grundy = new Array(maximum + 1); for(let i=0;i<maximum+1;i++) Grundy[i]=0; // Calculate Grundy Value of piles[i] and store it for (let i = 0; i <= n - 1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER1 piles Grundy n); /* Test Case 2 int piles[] = {3 8 2}; int n = sizeof(piles)/sizeof(piles[0]); int maximum = *max_element (piles piles + n); // An array to cache the sub-problems so that // re-computation of same sub-problems is avoided int Grundy [maximum + 1]; memset(Grundy -1 sizeof (Grundy)); // Calculate Grundy Value of piles[i] and store it for (int i=0; i<=n-1; i++) calculateGrundy(piles[i] Grundy); declareWinner(PLAYER2 piles Grundy n); */ // This code is contributed by avanitrachhadiya2155 </script>
Izlaz:
Player 1 will win
Vremenska složenost: O (n^2) gdje je n maksimalni broj kamenja u hrpi.
Složena složenost: O (n) Kako se Grundy niz koristi za pohranu rezultata podproblema kako bi se izbjegli suvišna računanja i potreban je O (n) prostor.
REFERENCE:
https://en.wikipedia.org/wiki/sprague%E2%80%93Grundy_Theorem
Vježbajte čitateljima: Razmislite o igri u nastavku.
Igra igraju dva igrača s N cijeli brojeve A1 A2 .. An. Na skretanju igrač odabire cijeli broj, dijeli ga s 2 3 ili 6, a zatim uzima pod. Ako cijeli broj postane 0, uklanja se. Posljednji igrač koji se kreće pobjeđuje. Koji igrač pobjeđuje u igri ako oba igrača igraju optimalno?
Savjet: Pogledajte primjer 3 od prethodni članak.