update 036

Author: andavid

README.md

@@ -24,6 +24,7 @@
 | 66   | [Plus One][066]                             |
 | 283  | [Move Zeroes][283]                          |
 | 1    | [Two Sum][001]                              |
+| 36   | [Valid Sudoku][036]                         |
 
 **其他**
 
@@ -47,3 +48,4 @@
 [066]: https://github.com/andavid/leetcode-java/blob/master/note/066/README.md
 [283]: https://github.com/andavid/leetcode-java/blob/master/note/283/README.md
 [001]: https://github.com/andavid/leetcode-java/blob/master/note/001/README.md
+[036]: https://github.com/andavid/leetcode-java/blob/master/note/036/README.md

note/036/README.md

@@ -0,0 +1,58 @@
+# [Valid Sudoku][title]
+
+## Description
+
+Determine if a Sudoku is valid, according to: [Sudoku Puzzles - The Rules][sudoku].
+
+The Sudoku board could be partially filled, where empty cells are filled with the character `'.'`.
+
+![](https://upload.wikimedia.org/wikipedia/commons/thumb/f/ff/Sudoku-by-L2G-20050714.svg/250px-Sudoku-by-L2G-20050714.svg.png)
+
+A partially filled sudoku which is valid.
+
+**Note:**
+A valid Sudoku board (partially filled) is not necessarily solvable. Only the filled cells need to be validated.
+
+## 思路
+每个数字，在每一行、每一列、每一个小九宫格内，只能出现一次。
+
+使用 3 个 9*9 的数组，记录每一行、每一列、每一个小九宫格里 1-9 出现的情况，如果某个数字出现，该将数字对应的位置置为 true。
+
+因此，只需要从左往右，从上到下，依次遍历 board 里的数字，一旦发现数字所在的行、列或小九宫格里该数字对应的位置为 true，说明不是有效的数独。
+
+## [完整代码][src]
+
+```java
+import java.util.HashSet;
+
+public class Solution {
+  public boolean isValidSudoku(char[][] board) {
+    if (board == null || board.length != 9 || board[0].length != 9) {
+      return false;
+    }
+
+    boolean[][] rows = new boolean[9][9];
+    boolean[][] cols = new boolean[9][9];
+    boolean[][] boxs = new boolean[9][9];
+
+    for (int i = 0; i < 9; i++) {
+      for (int j = 0; j < 9; j++) {
+        if (board[i][j] != '.') {
+          int k = (i / 3) * 3 + j / 3;
+          int number = board[i][j] - '0' - 1;
+          if (rows[i][number] || cols[j][number] || boxs[k][number]) {
+            return false;
+          }
+          rows[i][number] = cols[j][number] = boxs[k][number] = true;
+        }
+      }
+    }
+
+    return true;
+  }
+}
+```
+
+[title]: https://leetcode.com/problems/valid-sudoku
+[sudoku]: http://sudoku.com.au/TheRules.aspx
+[src]: https://github.com/andavid/leetcode-java/blob/master/src/com/andavid/leetcode/_036/Solution.java

src/com/andavid/leetcode/_036/Solution.java

@@ -2,34 +2,23 @@
 
 class Solution {
   public boolean isValidSudoku(char[][] board) {
-    final int size = 9;
-
-    if (board == null || board.length != size || board[0].length != size) {
+    if (board == null || board.length != 9 || board[0].length != 9) {
       return false;
     }
 
-    ArrayList<HashSet<Character>> row = new ArrayList<HashSet<Character>>();
-    ArrayList<HashSet<Character>> col = new ArrayList<HashSet<Character>>();
-    ArrayList<HashSet<Character>> area = new ArrayList<HashSet<Character>>();
-
-    for (int i = 0; i < size; i++) {
-      row.add(new HashSet<Character>());
-      col.add(new HashSet<Character>());
-      area.add(new HashSet<Character>());
-    }
+    boolean[][] rows = new boolean[9][9];
+    boolean[][] cols = new boolean[9][9];
+    boolean[][] boxs = new boolean[9][9];
 
-    for (int i = 0; i < size; i++) { // row
-      for (int j = 0; j < size; j++) { // column
-        int box = 3 * (i / 3) + j / 3; // sub box
+    for (int i = 0; i < 9; i++) {
+      for (int j = 0; j < 9; j++) {
         if (board[i][j] != '.') {
-          if (row.get(i).contains(board[i][j])
-              || col.get(j).contains(board[i][j])
-              || area.get(box).contains(board[i][j])) {
+          int k = (i / 3) * 3 + j / 3;
+          int number = board[i][j] - '0' - 1;
+          if (rows[i][number] || cols[j][number] || boxs[k][number]) {
             return false;
           }
-          row.get(i).add(board[i][j]);
-          col.get(j).add(board[i][j]);
-          area.get(box).add(board[i][j]);
+          rows[i][number] = cols[j][number] = boxs[k][number] = true;
         }
       }
     }