From 40f87e2da317814054308429e23afe48a7f5b995 Mon Sep 17 00:00:00 2001
From: Rakshita R Mirji <rakshitamirji77@gmail.com>
Date: Sat, 1 Nov 2025 19:36:30 +0530
Subject: [PATCH] Added Sudoku Solver algorithm in backtrackinh package

---
 .../backtracking/SudokuSolver.java            | 118 ++++++++++++++++++
 1 file changed, 118 insertions(+)
 create mode 100644 src/main/java/com/thealgorithms/backtracking/SudokuSolver.java

diff --git a/src/main/java/com/thealgorithms/backtracking/SudokuSolver.java b/src/main/java/com/thealgorithms/backtracking/SudokuSolver.java
new file mode 100644
index 000000000000..0ec4a1f0d8bf
--- /dev/null
+++ b/src/main/java/com/thealgorithms/backtracking/SudokuSolver.java
@@ -0,0 +1,118 @@
+package com.thealgorithms.backtracking;
+
+/**
+ * Program description - Solves a given 9x9 Sudoku puzzle using backtracking.
+ * <a href="https://en.wikipedia.org/wiki/Sudoku">Wikipedia</a>
+ *
+ * @author <a href="https://github.com/codewithRakshita">Rakshita</a>
+ */
+public class SudokuSolver {
+
+    private static final int SIZE = 9; // size of the Sudoku grid
+
+    /**
+     * Solves the Sudoku puzzle by filling empty cells.
+     *
+     * @param board The 9x9 Sudoku puzzle grid
+     * @return true if the Sudoku puzzle is solvable, false otherwise
+     */
+    public boolean solveSudoku(int[][] board) {
+        for (int row = 0; row < SIZE; row++) {
+            for (int col = 0; col < SIZE; col++) {
+
+                // Check for an empty cell
+                if (board[row][col] == 0) {
+
+                    // Try numbers 1 to 9
+                    for (int num = 1; num <= SIZE; num++) {
+                        if (isSafe(board, row, col, num)) {
+                            board[row][col] = num;
+
+                            // Recursively try filling the rest
+                            if (solveSudoku(board)) {
+                                return true;
+                            }
+
+                            // Backtrack
+                            board[row][col] = 0;
+                        }
+                    }
+
+                    // No valid number found — backtrack
+                    return false;
+                }
+            }
+        }
+
+        return true; // solved
+    }
+
+    /**
+     * Checks whether placing a number at a specific position is valid.
+     */
+    private boolean isSafe(int[][] board, int row, int col, int num) {
+        // Check row
+        for (int x = 0; x < SIZE; x++) {
+            if (board[row][x] == num) {
+                return false;
+            }
+        }
+
+        // Check column
+        for (int x = 0; x < SIZE; x++) {
+            if (board[x][col] == num) {
+                return false;
+            }
+        }
+
+        // Check 3x3 sub-grid
+        int startRow = row - row % 3;
+        int startCol = col - col % 3;
+
+        for (int i = 0; i < 3; i++) {
+            for (int j = 0; j < 3; j++) {
+                if (board[i + startRow][j + startCol] == num) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * Prints the Sudoku grid.
+     */
+    public void printBoard(int[][] board) {
+        for (int r = 0; r < SIZE; r++) {
+            for (int d = 0; d < SIZE; d++) {
+                System.out.print(board[r][d] + " ");
+            }
+            System.out.println();
+        }
+        System.out.println();
+    }
+
+    // Example usage (for testing)
+    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}
+        };
+
+        SudokuSolver solver = new SudokuSolver();
+        if (solver.solveSudoku(board)) {
+            System.out.println("Solved Sudoku:");
+            solver.printBoard(board);
+        } else {
+            System.out.println("No solution exists.");
+        }
+    }
+}