[leetcode]Sudoku Solver @ Python

原题地址：https://oj.leetcode.com/problems/sudoku-solver/

题意：

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.

解题思路：使用dfs来解决问题。

代码：

class Solution:

    # @param board, a 9x9 2D array

    # Solve the Sudoku by modifying the input board in-place.

    # Do not return any value.

    def solveSudoku(self, board):

        def isValid(x,y):

            tmp=board[x][y]; board[x][y]='D'

            for i in range(9):

                if board[i][y]==tmp: return False

            for i in range(9):

                if board[x][i]==tmp: return False

            for i in range(3):

                for j in range(3):

                    if board[(x/3)*3+i][(y/3)*3+j]==tmp: return False

            board[x][y]=tmp

            return True

        def dfs(board):

            for i in range(9):

                for j in range(9):

                    if board[i][j]=='.':

                        for k in '123456789':

                            board[i][j]=k

                            if isValid(i,j) and dfs(board):

                                return True

                            board[i][j]='.'

                        return False

            return True

        dfs(board)