BAEL-1298 Sudoku - Backtracking Algorithm

algorithms/src/main/java/com/baeldung/algorithms/sudoku/BacktrackingAlgorithm.java

@@ -0,0 +1,89 @@
+package com.baeldung.algorithms.sudoku;
+
+public class BacktrackingAlgorithm {
+
+    public static void main(String[] args) {
+        
+        char[][] board = { 
+            { '8', '.', '.', '.', '.', '.', '.', '.', '.' }, 
+            { '.', '.', '3', '6', '.', '.', '.', '.', '.' }, 
+            { '.', '7', '.', '.', '9', '.', '2', '.', '.' }, 
+            { '.', '5', '.', '.', '.', '7', '.', '.', '.' },
+            { '.', '.', '.', '.', '4', '5', '7', '.', '.' }, 
+            { '.', '.', '.', '1', '.', '.', '.', '3', '.' }, 
+            { '.', '.', '1', '.', '.', '.', '.', '6', '8' }, 
+            { '.', '.', '8', '5', '.', '.', '.', '1', '.' },
+            { '.', '9', '.', '.', '.', '.', '4', '.', '.' }
+        };
+        
+        BacktrackingAlgorithm solver = new BacktrackingAlgorithm();
+        solver.solve(board);
+
+        for (int i = 0; i < 9; i++) {
+            for (int j = 0; j < 9; j++) {
+                System.out.print(board[i][j] + " ");
+            }
+            System.out.println();
+        }
+    }
+    
+    public boolean solve(char[][] board) {
+        for (int r = 0; r < board.length; r++) {
+            for (int c = 0; c < board[0].length; c++) {
+                if (board[r][c] == '.') {
+                    for (int k = 1; k <= 9; k++) {
+                        board[r][c] = (char) ('0' + k);
+                        if (isValid(board, r, c) && solve(board)) {
+                            return true;
+                        } else {
+                            board[r][c] = '.';
+                        }
+                    }
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+    
+    public boolean isValid(char[][] board, int r, int c) {
+        //check row
+        boolean[] row = new boolean[9];
+        for (int i = 0; i < 9; i++) {
+            if (board[r][i] >= '1' && board[r][i] <= '9') {
+                if (row[board[r][i] - '1'] == false) {
+                    row[board[r][i] - '1'] = true;
+                } else {
+                    return false;
+                }
+            }
+        }
+
+        //check column
+        boolean[] col = new boolean[9];
+        for (int i = 0; i < 9; i++) {
+            if (board[i][c] >= '1' && board[i][c] <= '9') {
+                if (col[board[i][c] - '1'] == false) {
+                    col[board[i][c] - '1'] = true;
+                } else {
+                    return false;
+                }
+            }
+        }
+
+        //check the 3*3 grid
+        boolean[] grid = new boolean[9];
+        for (int i = (r / 3) * 3; i < (r / 3) * 3 + 3; i++) {
+            for (int j = (c / 3) * 3; j < (c / 3) * 3 + 3; j++) {
+                if (board[i][j] >= '1' && board[i][j] <= '9') {
+                    if (grid[board[i][j] - '1'] == false) {
+                        grid[board[i][j] - '1'] = true;
+                    } else {
+                        return false;
+                    }
+                }
+             }
+        }
+        return true;
+    }
+}