Added task 37.

Author: javadev

src/main/java/s0037_sudoku_solve/Solution.java

@@ -0,0 +1,64 @@
+package s0037_sudoku_solve;
+
+import java.util.ArrayList;
+import java.util.List;
+
+public class Solution {
+    List<int[]> emptyCells = new ArrayList<>();
+    int[] rows = new int[9];
+    int[] cols = new int[9];
+    int[] boxes = new int[9];
+
+    public void solveSudoku(char[][] board) {
+        for (int r = 0; r < 9; r++) {
+            for (int c = 0; c < 9; c++) {
+                if (board[r][c] == '.') {
+                    emptyCells.add(new int[] {r, c});
+                } else {
+                    int val = board[r][c] - '0';
+                    int boxPos = r / 3 * 3 + c / 3;
+                    rows[r] |= 1 << val;
+                    cols[c] |= 1 << val;
+                    boxes[boxPos] |= 1 << val;
+                }
+            }
+        }
+        backtracking(board, 0);
+    }
+
+    boolean backtracking(char[][] board, int i) {
+        // Check if we filled all empty cells?
+        if (i == emptyCells.size()) {
+            return true;
+        }
+
+        int r = emptyCells.get(i)[0];
+        int c = emptyCells.get(i)[1];
+        int boxPos = (r / 3) * 3 + c / 3;
+        for (int val = 1; val <= 9; ++val) {
+            // skip if that value is existed!
+            if (hasBit(rows[r], val) || hasBit(cols[c], val) || hasBit(boxes[boxPos], val)) {
+                continue;
+            }
+            board[r][c] = (char) ('0' + val);
+            // backup old values
+            int oldRow = rows[r];
+            int oldCol = cols[c];
+            int oldBox = boxes[boxPos];
+            rows[r] |= 1 << val;
+            cols[c] |= 1 << val;
+            boxes[boxPos] |= 1 << val;
+            if (backtracking(board, i + 1)) {
+                return true;
+            }
+            rows[r] = oldRow;
+            cols[c] = oldCol;
+            boxes[boxPos] = oldBox;
+        }
+        return false;
+    }
+
+    boolean hasBit(int x, int k) {
+        return (x >> k & 1) == 1;
+    }
+}

src/test/java/s0037_sudoku_solve/SolutionTest.java

@@ -0,0 +1,36 @@
+package s0037_sudoku_solve;
+
+import static org.hamcrest.CoreMatchers.equalTo;
+import static org.hamcrest.MatcherAssert.assertThat;
+
+import org.junit.Test;
+
+public class SolutionTest {
+    @Test
+    public void solveSudoku() {
+        char[][] expected = {
+            {'5', '3', '4', '6', '7', '8', '9', '1', '2'},
+            {'6', '7', '2', '1', '9', '5', '3', '4', '8'},
+            {'1', '9', '8', '3', '4', '2', '5', '6', '7'},
+            {'8', '5', '9', '7', '6', '1', '4', '2', '3'},
+            {'4', '2', '6', '8', '5', '3', '7', '9', '1'},
+            {'7', '1', '3', '9', '2', '4', '8', '5', '6'},
+            {'9', '6', '1', '5', '3', '7', '2', '8', '4'},
+            {'2', '8', '7', '4', '1', '9', '6', '3', '5'},
+            {'3', '4', '5', '2', '8', '6', '1', '7', '9'}
+        };
+        char[][] board = {
+            {'5', '3', '.', '.', '7', '.', '.', '.', '.'},
+            {'6', '.', '.', '1', '9', '5', '.', '.', '.'},
+            {'.', '9', '8', '.', '.', '.', '.', '6', '.'},
+            {'8', '.', '.', '.', '6', '.', '.', '.', '3'},
+            {'4', '.', '.', '8', '.', '3', '.', '.', '1'},
+            {'7', '.', '.', '.', '2', '.', '.', '.', '6'},
+            {'.', '6', '.', '.', '.', '.', '2', '8', '.'},
+            {'.', '.', '.', '4', '1', '9', '.', '.', '5'},
+            {'.', '.', '.', '.', '8', '.', '.', '7', '9'}
+        };
+        new Solution().solveSudoku(board);
+        assertThat(board, equalTo(expected));
+    }
+}