解决n皇后时的怪事

Question

So, I was solving n-queens problem and wrote this backtracking solution.

def isSafe(row, col, board):
    print board
    for i in range(col):
        if board[row][i] == 'Q':
            print 'faled here'
            return False

    r_row = row-1
    c_col = col-1
    while r_row >= 0 and c_col >=0:
        if board[c_row][c_col] == 'Q':
            return False
        c_row -=1
        c_col -=1

    row = row-1
    col = col-1
    while row < len(board) and col >=0:
        if board[row][col] == 'Q':
            return False
        row+=1
        col-=1
    return True


def solveNQueen(column, board):
    if column == len(board[0]):
        print board
        return True

    for each_row in range(len(board)):
        print each_row,column
        if isSafe(each_row,column,board):
            print board,'before'
            board[each_row][column] = 'Q'
            print board,' after'
            if solveNQueen(column+1,board):
                return True
            else:
                board[each_row][column] = 0
        print 'failed'
    return False

board = [[0]*5]*5

print solveNQueen(0,board)

The weird thing is line 34, 35 and 36 where I wrote:

    print board,'before'
    board[each_row][column] = 'Q'
    print board,' after'

This statement is changing all indexes in the same column to 'Q' instead of changing it at a particular row and column index.

From the output:

[[0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0], [0, 0, 0, 0, 0]] before
[['Q', 0, 0, 0, 0], ['Q', 0, 0, 0, 0], ['Q', 0, 0, 0, 0], ['Q', 0, 0, 0, 0], ['Q', 0, 0, 0, 0]]  after

Whats going on? Or am I just drunk?

Answer 1

The issue is board = [[0]*5]*5 . This gives you five copies of the same list of five zeros.

One possible fix:

board = [x[:] for x in [[0] * 5] * 5]