Leetcode 37. 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.



A sudoku puzzle...



...and its solution numbers marked in red.

这是数独的系列题中的一道，判断是否Valid, 同样使用遍历行遍历列，然后再遍历小方格。这道题还使用到的方法是DFS和回溯，这次题的回溯的方法是将曾经置位的数清掉，即装换为'.'

code is from


爱做饭的小莹子
http://www.cnblogs.com/springfor/p/3884252.html
public class Solution {
public void solveSudoku(char[][] board) {
if (board==null||board.length==0)
return;
helper(board);
}

private boolean helper(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 num='1'; num<='9'; num++){//尝试
if(isValid(board, i, j, num)){
board[i][j]=num;

if (helper(board))
return true;
else
board[i][j]='.';//回退
}
}
return false;
}
}
}

return true;
}

private boolean isValid(char[][] board, int i, int j, char c){
// check column
for (int row=0; row<9; row++)
if (board[row][j] == c)
return false;

// check row
for (int col=0; col<9; col++)
if (board[i][col]==c)
return false;

// check block
for(int row=i/3*3; row<i/3*3+3; row++)
for (int col=j/3*3; col<j/3*3+3; col++)
if (board[row][col]==c)
return false;

return true;
}
}