必须返回真值，拿到唯一解，否则会被撤销选择，

    1. class Solution:
    2.     def solveSudoku(self, board: List[List[str]]) -> None:
    3.         """
    4.         Do not return anything, modify board in-place instead.
    5.         """
    6.         def isvaild(i,j,num):
    7.             for x in range(0,9):
    8.                 if board[i][x]==str(num):return False
    9.                 if board[x][j]==str(num):return False
    10.                 if board[(i//3)*3+x//3][(j//3)*3+x%3]==str(num): return False
    11.             return True
    12.         def backtrack(i,j):
    13.             if i==9:return  #True
    14.             if j==9:
    15.                 return backtrack(i+1,0)
    16.             if board[i][j]!='.': backtrack(i,j+1)
    17.             if board[i][j]=='.':
    18.                 for num in range(1,10):
    19.                     if isvaild(i,j,num):
    20.                         board[i][j]=str(num)
    21.                         backtrack(i,j+1)#:return True
    22.                         print(num)
    23.                         board[i][j]='.'
    24.                 #return False
    25.         backtrack(0,0)