leetcode -- Sudoku Solver -- 经典重点

https://leetcode.com/problems/sudoku-solver/

思路就是dfs，这里可以把candidates子节点看做所有的点，然后判断if == ‘.’，也可以只scan 一遍9*9的board。

方法一

参考
http://www.cnblogs.com/zuoyuan/p/3770271.html

    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#当一个pos，1-9所有的digit都不满足要求的时候，就要return False，回溯到上一个candidates。            return True#如果所有的pos，所有的candidates都valid，那么return True        dfs(board)

方法二

http://jelices.blogspot.hk/2014/06/leetcode-python-sudoku-solver.html

还可以子节点只是下一个节点。 但是这个程序更慢，不知道为什么？

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):        self.solveSudokuRec(board,0,0)    def solveSudokuRec(self, board,row,col):        if row == 9:            return True        if col == 8:            nextRow = row +1            nextCol = 0        else:            nextRow = row            nextCol = col+1        if board[row][col]!='.':            return self.solveSudokuRec(board,nextRow,nextCol)        for c in range(1,10):            if self.canPut(board,str(c),row,col):                board[row][col] = str(c)                if self.solveSudokuRec(board,nextRow,nextCol):                    return True                board[row][col] = '.'        return False    def canPut(self, board, char, row, col):        for i in range(0,9):            if board[row][i] == char:                return False            if board[i][col] == char:                return False        rowGroup = (row//3) * 3        colGroup = (col//3) * 3         for i in range(rowGroup, rowGroup+3):            for j in range(colGroup, colGroup+3):                if board[i][j] == char:                    return False        return True