elegant stream solution

src/main/java/Solution.java

@@ -1,71 +1,59 @@
 import java.util.*;
+import java.util.function.IntFunction;
+import java.util.stream.*;
 
-/**
- * Template code to help you parse the standard input 
- * according to the problem statement.
- **/
 class Solution {
 	
+	private static final int SIZE = 9;
+	private static final int BLOCK_SIZE = 3;
+	
 	public static void main( String[] args ) {
-		Scanner in = new Scanner( System.in );
-		
-		int[][] grid = new int[9][9];
-		for ( int index = 0; in.hasNext(); index++ ) {
-			int row = index / 9;
-			int col = index % 9;
-			grid[row][col] = in.nextShort();
-		}
+		var digits = new Scanner( System.in ).tokens()
+				.map( Integer::valueOf )
+				.collect( Collectors.toList() );
 		
-		System.out.println( checkHorizontally( grid )
-		                    && checkVertically( grid )
-		                    && checkAllBlocks( grid )
-		);
+		System.out.println( checkSudoku( digits ));
+	}
+	
+	/**
+	 * The basic idea is to split the digits into groups that can be verified by the same logic over and over again.
+	 * The code will create 27 groups - 9 lines, 9 rows and 9 blocks - and checks for all digits in each group.
+	 */
+	private static boolean checkSudoku( List<Integer> digits ) {
+		return Stream.of(
+				filterGroup( digits, Solution::byLine ),
+				filterGroup( digits, Solution::byRow ),
+				filterGroup( digits, Solution::byBlock )
+		).flatMap( Collection::stream )
+		 .allMatch( Solution::checkAllDigits );
+	}
+	
+	/**
+	 * Splitting is done with a grouping function that calculates the group number for a grid index.
+	 */
+	private static Collection<List<Integer>> filterGroup( List<Integer> digits, IntFunction<Integer> grpFunc ) {
+		var group = IntStream.range( 0, digits.size() ).boxed()
+				.collect( Collectors.groupingBy( grpFunc::apply, Collectors.mapping( digits::get, Collectors.toList() )));
+		return group.values();
 	}
 	
-	private static boolean checkHorizontally( int[][] grid ) {
-		//noinspection ForLoopReplaceableByForEach
-		for ( int r = 0; r < grid.length; r++ ) {
-			BitSet bs = new BitSet( 9 );
-			for ( int c = 0; c < grid[r].length; c++ ) {
-				bs.set( grid[r][c] );
-			}
-			if ( bs.cardinality() != 9 ) return false;
-		}
-		return true;
+	/** 
+	 * Checks if every digit from 1 to 9 is present in each group.
+	 */
+	private static boolean checkAllDigits( List<Integer> group ) {
+		return group.stream().distinct().count() == SIZE;
 	}
 	
-	private static boolean checkVertically( int[][] grid ) {
-		for ( int c = 0; c < 9; c++ ) {
-			BitSet bs = new BitSet( 9 );
-			//noinspection ForLoopReplaceableByForEach
-			for ( int r = 0; r < grid.length; r++ ) {
-				bs.set( grid[r][c] );
-			}
-			if ( bs.cardinality() != 9 ) return false;
-		}
-		return true;
+	private static int byLine( int index ) {
+		return index / SIZE;
 	}
 	
-	private static boolean checkAllBlocks( int[][] grid ) {
-		return checkBlock( grid, 0, 0 )
-		       && checkBlock( grid, 0, 3 )
-		       && checkBlock( grid, 0, 6 )
-		       && checkBlock( grid, 3, 0 )
-		       && checkBlock( grid, 3, 3 )
-		       && checkBlock( grid, 3, 6 )
-		       && checkBlock( grid, 6, 0 )
-		       && checkBlock( grid, 6, 3 )
-		       && checkBlock( grid, 6, 6 );
+	private static int byRow( int index ) {
+		return index % SIZE;
 	}
 	
-	private static boolean checkBlock( int[][] block, int indexTop, int indexLeft ) {
-		BitSet bs = new BitSet( 9 );
-		for ( int r = indexTop; r < indexTop + 3; r++ ) {
-			for ( int c = indexLeft; c < indexLeft + 3; c++ ) {
-				bs.set( block[r][c] );
-			}
-		}
-		return ( bs.cardinality() == 9 );
+	private static int byBlock( int index ) {
+		return byRow( index ) / BLOCK_SIZE + ( index / ( SIZE * BLOCK_SIZE )) * BLOCK_SIZE;
 	}
 	
 }