37_SudokuSolver.py

class Solution(object):
    def solveSudoku(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        def isvalid(x, y, board):
            tmp = board[x][y]
            board[x][y] = 'X'
            for i in range(9):
                if board[i][y] == tmp: return False
            for j in range(9):
                if board[x][j] == 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, board) and dfs(board):
                                return True
                            board[i][j]='.'
                        return False
            return True
        dfs(board)