Python 我的数独解算器总是不返回任何结果

我编写了以下数独解算器：

# map 012 to 0, 345 to 1, 678 to 6
dico = dict()
for x in xrange(9) :
        dico[ x ] = ( x / 3 ) * 3
print dico


def candidates( a, i, j ) :
    """"Get possible values at position (i,j) in array a"""

    #
    i2 = dico[ i ]
    j2 = dico[ j ]

    #
    res = set( xrange(1,10) )
    # same row
    res -= set( a[ i ][ a[ i ] != 0 ] )
    # same col
    res -= set( a[ :, j ][ a[ :, j ] != 0 ] )
    # same 3x3 square
    res -= set( a[ i2:i2+3, j2:j2+3 ][ a[ i2:i2+3, j2:j2+3 ] != 0 ] )
    #
    return res


# 
def solve( a ) :
    """Sudoku solver"""

    # grid solved
    if np.sum( a == 0 ) == 0 :
        print "Grid solved"
        print a
        return a

    else :

        #
        # Focus on the 1st 0
        tmp_where = np.where( a == 0 )
        i, j = tmp_where[ 0 ][ 0 ], tmp_where[ 1 ][ 0 ] 

        #
        for e in candidates( a, i, j ) :
            #
            tmp = a.copy()
            tmp[ i, j ] = e
            #
            solve( tmp )

当我为初始网格调用

res=solve（a）

时，它会打印正确的解决方案

# this is a
[[2 0 0 0 8 0 3 0 0]
 [0 6 0 0 7 0 0 8 4]
 [0 3 0 5 0 0 2 0 9]
 [0 0 0 1 0 5 4 0 8]
 [0 0 0 0 0 0 0 0 0]
 [4 0 2 7 0 6 0 0 0]
 [3 0 1 0 0 7 0 4 0]
 [7 2 0 0 4 0 0 6 0]
 [0 0 4 0 1 0 0 0 3]]

# this is the console output
Grid solved
[[2 4 5 9 8 1 3 7 6]
 [1 6 9 2 7 3 5 8 4]
 [8 3 7 5 6 4 2 1 9]
 [9 7 6 1 2 5 4 3 8]
 [5 1 3 4 9 8 6 2 7]
 [4 8 2 7 3 6 9 5 1]
 [3 9 1 6 5 7 8 4 2]
 [7 2 8 3 4 9 1 6 5]
 [6 5 4 8 1 2 7 9 3]]

但是我不能使用

res

，因为它总是等于

None

！！发生了什么事

    res = solve(m)
    print res == None # output is always True

---

递归调用中没有return语句，这就是结果为None的原因

def solve( a ) :
    """Sudoku solver"""

    # grid solved
    if np.sum( a == 0 ) == 0 :
        print "Grid solved"
        print a
        return a

    else :

        #
        # Focus on the 1st 0
        tmp_where = np.where( a == 0 )
        i, j = tmp_where[ 0 ][ 0 ], tmp_where[ 1 ][ 0 ] 

        #
        for e in candidates( a, i, j ) :
            #
            tmp = a.copy()
            tmp[ i, j ] = e
            #
            res = solve( tmp )
            if res: #a failed path will return None, a success the solved matrix
                return res

---

你怎么称呼它？返回else分支？@Cjkjvfnby不，这会阻止探索所有候选对象，请注意它位于的

中。