Combinatoriale speltheorie | Set 4 (Sprague - Grundy Stelling)
Vereisten: Grundy nummers/cijfers en mex
We hebben al gezien in set 2 (https://www.geeksforgeeks.org/dsa/combinatorial-game-theory-set-2-game-nim/) dat we kunnen vinden wie wint in een spel van NIM zonder de game daadwerkelijk te spelen.
Stel dat we het klassieke NIM -spel een beetje veranderen. Deze keer kan elke speler alleen 1 2 of 3 stenen verwijderen (en niet een willekeurig aantal stenen zoals in het klassieke spel van NIM). Kunnen we voorspellen wie er zal winnen?
Ja, we kunnen de winnaar voorspellen met behulp van Sprague-Grundy Stelling.
Wat is Sprague-Grundy Stelling?
Stel dat er een samengestelde game (meer dan één sub-game) bestaat uit n sub-games en twee spelers A en B. Vervolgens zegt Sprague-Grundy Stelling dat als zowel A als B optimaal spelen (d.w.z. ze geen fouten maken), de speler als eerste wordt gegarandeerd om te winnen als de XOR van de Grundy-getallen van de Sub-games in het begin van de game niet-zaal is. Anders als de XOR evalueert naar nul, verliest speler A zeker wat er ook gebeurt.
Hoe breng je Sprague Grundy Stelling aan?
We kunnen Sprague-Grundy Stelling in elk onpartijdig spel en los het op. De basisstappen worden als volgt vermeld:
- Breek het samengestelde spel in sub-games.
- Bereken vervolgens voor elke sub-game het Grundy-nummer op die positie.
- Bereken vervolgens de XOR van alle berekende Grundy -getallen.
- Als de XOR-waarde niet nul is, dan zal de speler die de beurt maakt (eerste speler) anders winnen, anders is hij voorbestemd om te verliezen, wat er ook gebeurt.
Voorbeeldspel: Het spel begint met 3 stapels met 3 4 en 5 stenen en de speler om te bewegen, kunnen elk positief aantal stenen tot 3 nemen van een van de palen [op voorwaarde dat de stapel zoveel stenen heeft]. De laatste speler die wint. Welke speler wint het spel, ervan uitgaande dat beide spelers optimaal spelen?
Hoe te vertellen wie er zal winnen door Sprague-Grundy Stelling toe te passen?
Zoals we kunnen zien dat deze game zelf bestaat uit verschillende sub-games.
Eerste stap: De sub-games kunnen als elke stapels worden beschouwd.
Tweede stap: We zien uit de onderstaande tabel dat
Grundy(3) = 3 Grundy(4) = 0 Grundy(5) = 1
We hebben al gezien hoe we de Grundy -nummers van dit spel kunnen berekenen in de vorig artikel.
Derde stap: De XOR van 3 0 1 = 2
Vierde stap: Omdat Xor een niet-nul nummer is, kunnen we zeggen dat de eerste speler zal winnen.
Hieronder is het programma dat boven 4 stappen implementeert.
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 using namespace std ; /* 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*/ #define PLAYER1 1 #define PLAYER2 2 // A Function to calculate Mex of all the values in that set int calculateMex ( unordered_set < int > Set ) { int Mex = 0 ; while ( Set . find ( Mex ) != Set . end ()) Mex ++ ; return ( Mex ); } // A function to Compute Grundy Number of 'n' 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 ]); unordered_set < int > Set ; // A Hash Table for ( int i = 1 ; i <= 3 ; i ++ ) Set . insert ( calculateGrundy ( n - i Grundy )); // Store the result Grundy [ n ] = calculateMex ( Set ); return ( Grundy [ n ]); } // A function to declare the winner of the game 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 ) printf ( 'Player 1 will win n ' ); else printf ( 'Player 2 will win n ' ); } else { if ( whoseTurn == PLAYER1 ) printf ( 'Player 2 will win n ' ); else printf ( 'Player 1 will win n ' ); } return ; } // Driver program to test above functions int main () { // Test Case 1 int piles [] = { 3 4 5 }; int n = sizeof ( piles ) / sizeof ( piles [ 0 ]); // Find the maximum element 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 ( 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); */ return ( 0 ); }
Java 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
Python3 ''' 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 win n ' ); else : print ( 'Player 2 will win n ' ); else : if ( whoseTurn == PLAYER1 ): print ( 'Player 2 will win n ' ); else : print ( 'Player 1 will win n ' ); # 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
C# 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
JavaScript < 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 ( n Grundy ) { 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 ( whoseTurn piles Grundy n ) { 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>
Uitvoer:
Player 1 will win
Tijdcomplexiteit: O (n^2) waarbij n het maximale aantal stenen in een stapel is.
Space Complexiteit: O (n) omdat de Grundy -array wordt gebruikt om de resultaten van subproblemen op te slaan om overbodige berekeningen te voorkomen en het duurt O (n) ruimte.
Referenties:
https://en.wikipedia.org/wiki/sprague%E2%80%93Grundy_Theorem
Oefening aan de lezers: Overweeg het onderstaande spel.
Een spel wordt gespeeld door twee spelers met N Integers A1 A2 .. An. Op zijn/haar beurt selecteert een speler een geheel getal verdeelt het met 2 3 of 6 en neemt vervolgens de vloer. Als het geheel getal 0 wordt, wordt het verwijderd. De laatste speler die wint. Welke speler wint het spel als beide spelers optimaal spelen?
Hint: zie het voorbeeld 3 van vorig artikel.
