Java for LeetCode 037 Sudoku Solver

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.

解题思路：

经典的NP问题，采用

Dancing Links

可以优化算法，参考链接：https://www.ocf.berkeley.edu/~jchu/publicportal/sudoku/sudoku.paper.html

本题方法多多，优化算法也是多多，本例仅给出最简单的DFS暴力枚举算法。

JAVA实现如下：

static public void solveSudoku(char[][] board) {
int count = 0;
for (int i = 0; i < board.length; i++)
for (int j = 0; j < board[0].length; j++)
if (board[i][j] == '.')
count++;
dfs(board, count);
}

public static int dfs(char[][] board, int count) {
if (count == 0)
return 0;
for (int i = 0; i < board.length; i++) {
for (int j = 0; j < board[0].length; j++) {
if (board[i][j] == '.') {
for (int k = 1; k <= 10; k++) {
if (k == 10)
return count;
board[i][j] = (char) ('0' + k);
if (!isValid(board, i, j))
board[i][j] = '.';
else {
count--;
count = dfs(board, count);
if (count == 0)
return count;
count++;
board[i][j] = '.';
}
}
}
}
}
return count;
}

static public boolean isValid(char[][] board, int row, int col) {
HashMap<Character, Integer> hashmap = new HashMap<Character, Integer>();
for (int j = 0; j < board[0].length; j++) {
if (board[row][j] != '.') {
if (hashmap.containsKey(board[row][j]))
return false;
hashmap.put(board[row][j], 1);
}
}

hashmap = new HashMap<Character, Integer>();
for (int i = 0; i < board.length; i++) {
if (board[i][col] != '.') {
if (hashmap.containsKey(board[i][col]))
return false;
hashmap.put(board[i][col], 1);
}
}

hashmap = new HashMap<Character, Integer>();
int rowTemp = (row / 3) * 3;
int colTemp = (col / 3) * 3;

for (int k = 0; k < 9; k++) {
if (board[rowTemp + k / 3][colTemp + k % 3] != '.') {
if (hashmap
.containsKey(board[rowTemp + k / 3][colTemp + k % 3]))
return false;
hashmap.put(board[rowTemp + k / 3][colTemp + k % 3], 1);
}
}
return true;
}