[leetcode]Sudoku Solver

DFS。这种思路清晰的题目，大问题一般出在小地方。

1.当DFS找到时，要返回true。这样一路true上去，否则最后格子又会被设为'.'；

2.计算格子所在九宫格时，要用int a = x/3*3+i; 忘记乘3了，一直错；

public class Solution {
    public void solveSudoku(char[][] board) {
        // Start typing your Java solution below
        // DO NOT write main() function
        int m = board.length;
        if (m == 0) return;
        int n = board[0].length;
        if (n == 0) return;       
        
        ArrayList<Point> arr = new ArrayList<Point>();
        
        for (int i = 0; i < m; i++) {
            for (int j = 0; j < n; j++) {
                if (board[i][j] == '.') {
                    Point p = new Point();
                    p.x = i;
                    p.y = j;
                    arr.add(p);
                }
            }
        }
        
        dfs(board, arr, 0);
    }
        
    boolean dfs(char[][] board, ArrayList<Point> arr, int k) {
        if (k == arr.size()) {
            return true;
        }
        else {
            for (int d = 0; d < 9; d++) {
                Point p = arr.get(k);
                board[p.x][p.y] = (char)(d + '1');
                // check valid
                if (IsValid(board, p.x, p.y) && dfs(board, arr, k+1))
                {
                    return true;
                }
                board[p.x][p.y] = '.';                    
            }
        }
        return false;
    }
    
    private boolean IsValid(char[][] board, int x, int y) {
        int len = board.length;
        char c = board[x][y];
        for (int i = 0; i < len; i++) {
            if (x != i && board[i][y] == c) {
                return false;
            }
        }
        
        for (int i = 0; i < len; i++) {
            if (y != i && board[x][i] == c) {
                return false;
            }
        }
        
        for (int i = 0; i < 3; i++) {
            for (int j = 0; j < 3; j++) {
                int a = x/3*3+i;
                int b = y/3*3+j;
                if ((a != x) && (b != y) && board[a][b] == c) {
                    return false;
                }
            }
        }
        return true;
    }
}

class Point {
    public int x;
    public int y;
}