transform-to-chessboard.py

# Time:  O(n^2)
# Space: O(n^2), used by Counter, this could be reduced to O(n) by skipping invalid input

# An N x N board contains only 0s and 1s. In each move, you can swap any 2 rows with each other,
# or any 2 columns with each other.
#
# What is the minimum number of moves to transform the board into a "chessboard" -
# a board where no 0s and no 1s are 4-directionally adjacent? If the task is impossible, return -1.
#
# Examples:
# Input: board = [[0,1,1,0],[0,1,1,0],[1,0,0,1],[1,0,0,1]]
# Output: 2
# Explanation:
# One potential sequence of moves is shown below, from left to right:
#
# 0110     1010     1010
# 0110 --> 1010 --> 0101
# 1001     0101     1010
# 1001     0101     0101
#
# The first move swaps the first and second column.
# The second move swaps the second and third row.
#
# Input: board = [[0, 1], [1, 0]]
# Output: 0
# Explanation:
# Also note that the board with 0 in the top left corner,
# 01
# 10
#
# is also a valid chessboard.
#
# Input: board = [[1, 0], [1, 0]]
# Output: -1
# Explanation:
# No matter what sequence of moves you make, you cannot end with a valid chessboard.
#
# Note:
# - board will have the same number of rows and columns, a number in the range [2, 30].
# - board[i][j] will be only 0s or 1s.

class Solution(object):
    def movesToChessboard(self, board):
        """
        :type board: List[List[int]]
        :rtype: int
        """
        N = len(board)
        result = 0
        for count in (collections.Counter(map(tuple, board)), \
                      collections.Counter(itertools.izip(*board))):
            if len(count) != 2 or \
               sorted(count.values()) != [N/2, (N+1)/2]:
                return -1

            seq1, seq2 = count
            if any(x == y for x, y in itertools.izip(seq1, seq2)):
                return -1
            begins = [int(seq1.count(1) * 2 > N)] if N%2 else [0, 1]
            result += min(sum(int(i%2 != v) for i, v in enumerate(seq1, begin)) \
                          for begin in begins) / 2
        return result