LeetCode 116 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.

分析：

这也是一个递归回溯问题。

public class Solution {
    public void solveSudoku(char[][] board) {
        if(board == null || board.length != 9 || board[0].length != 9 ) return;
        dfs(board, 0, 0);
    }
    public boolean dfs(char[][] board, int i, int j){
        if(j>=9)
            return dfs(board, i+1, 0);//下一行
        if(i==9)
            return true;
        if(board[i][j]=='.'){
            for(int k=1; k<=9; k++){
                board[i][j] = (char)(k+'0');
                if(isValid(board, i, j))
                    if(dfs(board, i, j+1))
                        return true;
                board[i][j] = '.';
            }
        }else
            return dfs(board, i, j+1);
        return false;
    }
    //判断当前放置是否合法
    public boolean isValid(char[][] board, int i, int j){
        //行
        for(int k=0; k<9; k++){
            if(k!=j && board[i][k]==board[i][j])
                return false;
        }
        //列
        for(int k=0; k<9; k++){
            if(k!=i && board[k][j]==board[i][j])
                return false;
        }
        //所在的九宫格
        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((row!=i||col!=j) && board[row][col]==board[i][j])
                    return false;
            }
        }
        return true;
    }
}