[LEET-37] Sudoko Solver

Author: fishercoder1534

leetcode-algorithms/README.md

@@ -278,6 +278,7 @@
 |40|[Combination Sum II](https://leetcode.com/problems/combination-sum-ii/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/CombinationSumII.java)|O(k*n^k)|O(k)|Medium|Backtracking
 |39|[Combination Sum](https://leetcode.com/problems/combination-sum/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/CombinationSum.java)|O(k*n^k)|O(k)|Medium|Backtracking
 |38|[Count and Say](https://leetcode.com/problems/count-and-say/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/CountandSay.java)|O(n*2^n)|O(2^n)|Easy| Recursion, LinkedList
+|37|[Sudoku Solver](https://leetcode.com/problems/sudoku-solver/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/SudokuSolver.java)|O((9!)^9)|O(1)|Hard|
 |36|[Valid Sudoku](https://leetcode.com/problems/valid-sudoku/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/ValidSudoku.java)|O(?)|O(?)|Medium| 
 |35|[Search Insert Position](https://leetcode.com/problems/search-insert-position/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/SearchInsertPosition.java)|O(n)|O(1)|Medium|Array
 |34|[Search for a Range](https://leetcode.com/problems/search-for-a-range/)|[Solution](../../master/leetcode-algorithms/src/main/java/com/stevesun/solutions/SearchForARange.java)|O(logn)|O(1)|Medium|Array, Binary Search

leetcode-algorithms/src/main/java/com/stevesun/solutions/SudokuSolver.java

@@ -0,0 +1,46 @@
+package com.stevesun.solutions;
+
+/**
+ * Write a program to solve a Sudoku puzzle by filling the empty cells.
+ * Empty cells are indicated by the character '.'.
+ * You may assume that there will be only one unique solution.
+  * A sudoku puzzle...
+ * ...and its solution numbers marked in red.
+ */
+public class SudokuSolver {
+
+    public void solveSudoku(char[][] board) {
+        if (board == null || board.length == 0) return;
+        solve(board);
+    }
+
+    private boolean solve(char[][] board) {
+        for (int i = 0; i < board.length; i++) {
+            for (int j = 0; j < board[0].length; j++) {
+                if (board[i][j] == '.') {
+                    for (char c = '1'; c <= '9'; c++) {//try 1 to 9
+                        if (isValid(board, i, j, c)) {
+                            board[i][j] = c;
+
+                            if (solve(board)) return true;
+                            else board[i][j] = '.';//recover it to be '.'
+                        }
+                    }
+                    return false;
+                }
+            }
+        }
+        return true;
+    }
+
+    private boolean isValid(char[][] board, int row, int col, char c) {
+        for (int i = 0; i < 9; i++) {
+            if (board[i][col] != '.' && board[i][col] == c) return false;//check row
+            if (board[row][i] != '.' && board[row][i] == c) return false;//check column
+            if (board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] != '.' && board[3 * (row / 3) + i / 3][3 * (col / 3) + i % 3] == c)
+                return false; //check 3*3 block
+        }
+        return true;
+    }
+
+}