Add sudoku solver with Backtracking Algorithm.

src/main/java/com/thealgorithms/backtracking/SudokuSolver.java

@@ -0,0 +1,74 @@
+package main.java.com.thealgorithms.backtracking;
+
+public class SudokuSolver {
+
+    public static boolean sudokuSolver(int[][] sudoku, int row, int col) {
+        if (row == 9) {
+            return true; // base case: reached the end
+        }
+
+        int nextRow = row;
+        int nextCol = col + 1;
+        if (nextCol == 9) {
+            nextRow = row + 1;
+            nextCol = 0;
+        }
+
+        if (sudoku[row][col] != 0) {
+            return sudokuSolver(sudoku, nextRow, nextCol);
+        }
+
+        for (int num = 1; num <= 9; num++) {
+            if (isSafe(sudoku, row, col, num)) {
+                sudoku[row][col] = num;
+                if (sudokuSolver(sudoku, nextRow, nextCol)) {
+                    return true;
+                }
+                sudoku[row][col] = 0; // backtrack
+            }
+        }
+        return false;
+    }
+
+    private static boolean isSafe(int[][] sudoku, int row, int col, int num) {
+        // Check row and column
+        for (int i = 0; i < 9; i++) {
+            if (sudoku[row][i] == num || sudoku[i][col] == num) {
+                return false;
+            }
+        }
+
+        // Check 3x3 subgrid
+        int startRow = row - row % 3;
+        int startCol = col - col % 3;
+        for (int i = startRow; i < startRow + 3; i++) {
+            for (int j = startCol; j < startCol + 3; j++) {
+                if (sudoku[i][j] == num) {
+                    return false;
+                }
+            }
+        }
+
+        return true;
+    }
+
+    private static void printSudoku(int[][] sudoku) {
+        for (int i = 0; i < 9; i++) {
+            for (int j = 0; j < 9; j++) {
+                System.out.print(sudoku[i][j] + " ");
+            }
+            System.out.println();
+        }
+    }
+
+    public static void main(String[] args) {
+        int[][] sudoku = {{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}};
+
+        if (sudokuSolver(sudoku, 0, 0)) {
+            System.out.println("Sudoku Solved:");
+            printSudoku(sudoku);
+        } else {
+            System.out.println("No solution exists.");
+        }
+    }
+}