*LeetCode-Sudoku Solver

一开始的代码 helper是void 这样就会形成填了一遍 发现不对 就不再往下填了

helper需要是返回boolean 这样再填到最后 发现填完了 就返回true； 假如上一层返回false 并且这一层的1-9全都填完了还没返回true 那说明这一层怎么填都不行 就把这一层设置称为原来的'.' 并返回给上一层false；

public class Solution {
    public void solveSudoku(char[][] board) {
        int newRow = 0;
        int newCol = 0;
        while ( board[newRow][newCol] != '.' ){
            newCol ++;
            if ( newCol >= 9 ){
                newRow ++;
                newCol = 0;
            }
        }        
        helper ( board, newRow, newCol );
    }
    public boolean helper ( char[][] board, int row, int col ){
        if ( row >= 9 )
            return true;
        for ( int i = 1; i <= 9; i ++ ){
            if ( valid ( board, row, col, i ) ){
                board [row][col] = (char)(i + '0');;
                int newRow = row;
                int newCol = col;
                while ( board[newRow][newCol] != '.' ){
                    newCol ++;
                    if ( newCol >= 9 ){
                        newRow ++;
                        newCol = 0;
                    }
                    if ( newRow >=9 )
                        break;
                }
                if ( helper ( board, newRow, newCol ) )
                    return true;
                board [row][col] = '.';
            }
        }
        return false;
    }
    
    public boolean valid ( char [][] board, int row, int col, int n) {
        //check row
        for ( int i = 0; i < 9; i ++ ){
            if ( board[row][i] == (char)(n + '0') && i != col )
                return false;
        }
        //check col
        for ( int i = 0; i < 9; i ++ ){
            if ( board[i][col] == (char)(n + '0') && i != row )
                return false;
        }        
        //check square
        for ( int i = row/3 * 3; i < (row/3 + 1) * 3; i ++ ){
            for ( int j = col/3 * 3; j < (col/3 + 1) * 3; j ++ ){
                if ( board[i][j] == (char)(n + '0') && i != row && j != col )
                    return false; 
            }
        }
        return true;
    }  
}