Refactor Sudoku Solver for Improved Readability and Maintainability (#3566)

Author: fineanmol

JavaSolutions/SudokoSolver.java

@@ -1,14 +1,20 @@
 public class SudokuSolver {
+
+    private static final int SIZE = 9;
+
     public static boolean solve(int[][] board) {
-        for (int i = 0; i < 9; i++) {
-            for (int j = 0; j < 9; j++) {
-                if (board[i][j] == 0) {
-                    for (int num = 1; num <= 9; num++) {
-                        if (isSafe(board, i, j, num)) {
-                            board[i][j] = num;
-                            if (solve(board))
+        for (int row = 0; row < SIZE; row++) {
+            for (int col = 0; col < SIZE; col++) {
+                if (board[row][col] == 0) {
+                    for (int num = 1; num <= SIZE; num++) {
+                        if (isSafe(board, row, col, num)) {
+                            board[row][col] = num;
+
+                            if (solve(board)) {
                                 return true;
-                            board[i][j] = 0; // backtrack
+                            }
+
+                            board[row][col] = 0; // Backtrack
                         }
                     }
                     return false;
@@ -19,35 +25,54 @@ public static boolean solve(int[][] board) {
     }
 
     private static boolean isSafe(int[][] board, int row, int col, int num) {
-        for (int k = 0; k < 9; k++)
-            if (board[row][k] == num || board[k][col] == num)
+        // Check row and column
+        for (int i = 0; i < SIZE; i++) {
+            if (board[row][i] == num || board[i][col] == num) {
                 return false;
+            }
+        }
+
+        // Check 3x3 subgrid
+        int startRow = row - row % 3;
+        int startCol = col - col % 3;
 
-        int sr = row - row % 3, sc = col - col % 3;
-        for (int r = 0; r < 3; r++)
-            for (int c = 0; c < 3; c++)
-                if (board[sr + r][sc + c] == num)
+        for (int r = 0; r < 3; r++) {
+            for (int c = 0; c < 3; c++) {
+                if (board[startRow + r][startCol + c] == num) {
                     return false;
+                }
+            }
+        }
 
         return true;
     }
 
+    private static void printBoard(int[][] board) {
+        for (int[] row : board) {
+            for (int num : row) {
+                System.out.print(num + " ");
+            }
+            System.out.println();
+        }
+    }
+
     public static void main(String[] args) {
         int[][] board = {
-            {5,3,0,0,7,0,0,0,0},
-            {6,0,0,1,9,5,0,0,0},
-            {0,9,8,0,0,0,0,6,0},
-            {8,0,0,0,6,0,0,0,3},
-            {4,0,0,8,0,3,0,0,1},
-            {7,0,0,0,2,0,0,0,6},
-            {0,6,0,0,0,0,2,8,0},
-            {0,0,0,4,1,9,0,0,5},
-            {0,0,0,0,8,0,0,7,9}
+            {5, 3, 0, 0, 7, 0, 0, 0, 0},
+            {6, 0, 0, 1, 9, 5, 0, 0, 0},
+            {0, 9, 8, 0, 0, 0, 0, 6, 0},
+            {8, 0, 0, 0, 6, 0, 0, 0, 3},
+            {4, 0, 0, 8, 0, 3, 0, 0, 1},
+            {7, 0, 0, 0, 2, 0, 0, 0, 6},
+            {0, 6, 0, 0, 0, 0, 2, 8, 0},
+            {0, 0, 0, 4, 1, 9, 0, 0, 5},
+            {0, 0, 0, 0, 8, 0, 0, 7, 9}
         };
-        solve(board);
-        for (int[] row : board) {
-            for (int num : row) System.out.print(num + " ");
-            System.out.println();
+
+        if (solve(board)) {
+            printBoard(board);
+        } else {
+            System.out.println("No solution exists.");
         }
     }
 }