题目描述：
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.
解题思路：
本题使用回溯和HashSet的方法。对于每一个空白位置，试探的使用‘1’-‘9’之间的数字，如果加入该数字在满足数独规则的情况下，将该字符加入该位置，向下去试探；如果试探失败，则回到当前这步，换另一个字符继续进行试探。
代码如下：
public class Solution {
public void solveSudoku(char[][] board) {
HashSet[] row = new HashSet[9];
HashSet[] col = new HashSet[9];
HashSet[] cell = new HashSet[9];
initHashSet(board, row, col, cell);
solve(board, row, col, cell);
}
public boolean solve(char[][] board, HashSet[] row, HashSet[] col,
HashSet[] cell) {
for (int i = 0; i < 9; i++) {
for (int j = 0; j < 9; j++) {
if (board[i][j] == ‘.‘) {
for (char c = ‘1‘; c <= ‘9‘; c++) {
if (isValidSudoku(board, i, j, c, row, col, cell)) {
board[i][j] = c;
row[i].add(c);
col[j].add(c);
cell[3 * (i / 3) + j / 3].add(c);
if (solve(board, row, col, cell))
return true;
else {
board[i][j] = ‘.‘;
row[i].remove(c);
col[j].remove(c);
cell[3 * (i / 3) + j / 3].remove(c);
}
}
}
return false;
}
}
}
return true;
}
public boolean isValidSudoku(char[][] board, int i, int j, char c,
HashSet[] row, HashSet[] col, HashSet[] cell) {
if (row[i].contains(c) || col[j].contains(c)
|| cell[3 * (i / 3) + j / 3].contains(c))
return false;
return true;
}
public void initHashSet(char[][] board, HashSet[] row,
HashSet[] col, HashSet[] cell) {
for (int i = 0; i < 9; i++) {
row[i] = new HashSet<Character>();
col[i] = new HashSet<Character>();
cell[i] = new HashSet<Character>();
}
for (int i = 0; i < 9; i++) {
for (int j = 0; j < 9; j++) {
if (board[i][j] != ‘.‘) {
row[i].add(board[i][j]);
col[j].add(board[i][j]);
cell[3 * (i / 3) + j / 3].add(board[i][j]);
}
}
}
}
}
Java [leetcode 37]Sudoku Solver
原文：http://www.cnblogs.com/zihaowang/p/4970069.html