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.
这题是Valid Sudoku的follow up。
思路还是DFS，需要注意的是如何check，需要每添加一个元素就检查当前元素所处的行和列，和其所在的3*3的小分块，如果没有冲突则继续。否则重新置为‘.’。值得注意的是可能因为之前的选择不对，会遇到当前‘.‘枚举所有元素都不合格的情况，此处会不断返回False，backtrack将之前所有加的点都弹出。
代码如下：
class Solution(object): def solveSudoku(self, board): """ :type board: List[List[str]] :rtype: void Do not return anything, modify board in-place instead. """ if not board or len(board)!= 9 or len(board[0]) != 9: return self.helper(board,0,0) return def helper(self, board, i, j): #i,j is the index of current position if j >= 9: return self.helper(board, i+1, 0) if i == 9: return True if board[i][j] == ‘.‘: for k in xrange(1,10): board[i][j] = str(k) if self.isValid(board, i, j): if self.helper(board, i, j+1): return True board[i][j] = ‘.‘ else: return self.helper(board, i, j+1) return False def isValid(self, board, i, j): for k in xrange(9): if k != j and board[i][k] == board[i][j]: return False for k in xrange(9): if k != i and board[k][j] == board[i][j]: return False for row in xrange(i/3*3, i/3*3+3): for col in xrange(j/3*3, j/3*3+3): if (row != i or col != j) and board[row][col] == board[i][j]: return False return True
原文：http://www.cnblogs.com/sherylwang/p/5819725.html