sudoku

Recursion/Sudoku.java

@@ -0,0 +1,80 @@
+import java.util.ArrayList;
+
+public class Sudoku {
+
+//    Given a partially filled two-dimensional array, fill all the unfilled cells such
+//    that each row, each column and each 3 x 3 subgrid (as highlighted below by bolder lines) has every digit from 1 to 9 exactly once.
+//    Unfilled cells have a value of 0 on the given board.
+
+    public static ArrayList<ArrayList<Integer>> solve_sudoku_puzzle(ArrayList<ArrayList<Integer>> board) {
+        helper(board);
+        return board;
+    }
+
+    private static boolean helper(ArrayList<ArrayList<Integer>> board) {
+
+        int row = 0;
+        int col = 0;
+        boolean unfilledCell = false;
+        for (int i = 0; i < board.size(); i++) {
+            for (int j = 0; j < board.get(0).size(); j++) {
+                if (board.get(i).get(j) == 0) {
+                    row = i;
+                    col = j;
+                    unfilledCell = true;
+                    break;
+                }
+                if (unfilledCell) {
+                    break;
+                }
+            }
+        }
+        if (!unfilledCell) {
+            return true;
+        }
+        for (int i = 1; i <= 9; i++) {
+            if (isSafe(board, row, col, i)) {
+                board.get(row).set(col, i);
+                if (helper(board)) {
+                    return true;
+                } else {
+                    board.get(row).set(col, 0);
+                }
+            }
+
+        }
+        return false;
+    }
+
+    static boolean isSafe(ArrayList<ArrayList<Integer>> board, int row, int col, int val) {
+        for (int i = 0; i < 9; i++) {
+            if (board.get(row).get(i) == val) {
+                return false;
+            }
+        }
+
+        for (int i = 0; i < 9; i++) {
+            if (board.get(i).get(col) == val) {
+                return false;
+            }
+        }
+
+        int rowStart = row - row % 3;
+        int colStart = col - col % 3;
+
+        for (int i = rowStart; i < rowStart + 3; i++) {
+            for (int j = colStart; j < colStart + 3; j++) {
+                if (board.get(i).get(j) == val) {
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+}
+
+
+
+
+
+