【leetcode】37. Sudoku Solver

题目如下：

解题思路：也没啥好说的，回溯法。

代码如下：

class Solution(object):
    length = 9
    def isValid(self,row,column,val,grid):
        #check row,column
        for i in range(self.length):
            if val == grid[row][i] and i != column:
                return False
            if val == grid[i][column] and i != row:
                return False
        #check self grid
        r = (row - row % 3)
        c = (column - column%3)
        for i in range(r,r+3):
            for j in range(c,c+3):
                if grid[i][j] == val and i != row and j != column:
                    return False
        return True
    def getValid(self,x,y,grid):
        if grid[x][y] == str(self.length):
            return -1
        elif grid[x][y] == '.':
            start = 1
        else:
            start = int(grid[x][y]) + 1
        for i in range(start,self.length+1):
            if self.isValid(x,y,str(i),grid) == True:
                return i
        return -1
    def getNextInx(self,i,j):
        if j < self.length-1:
            j += 1
        else:
            i += 1
            j = 0
        return i,j
    def solveSudoku(self, board):
        """
        :type board: List[List[str]]
        :rtype: void Do not return anything, modify board in-place instead.
        """
        stack = []
        i = j = 0
        while i < self.length:
            while j < self.length and i < self.length:
                if board[i][j] != '.':
                    i,j = self.getNextInx(i,j)
                    continue
                while True:
                    #print board[0]
                    flag = False
                    v  = self.getValid(i,j,board)
                    if v != -1:
                        board[i][j] = str(v)
                        stack.append((i, j))
                        flag = True
                    if flag == False:
                        while len(stack) > 0:
                            x,y = stack[-1]
                            v = self.getValid(x,y,board)
                            if v != -1:
                                board[x][y] = str(v)
                                flag = True
                                i = x
                                j = y
                                break
                            else:
                                board[x][y] = '.'
                                del stack[-1]
                        ## if flag still false,invalid sudoku
                        if flag == False:
                            return
                        break
                    else:
                        break
                i, j = self.getNextInx(i, j)