brick-wall.py

# Time:  O(n), n is the total number of the bricks
# Space: O(m), m is the total number different widths

# There is a brick wall in front of you. The wall is rectangular and has several rows of bricks.
# The bricks have the same height but different width. You want to draw a vertical line from
# the top to the bottom and cross the least bricks.
#
# The brick wall is represented by a list of rows. Each row is a list of integers representing the
# width of each brick in this row from left to right.
#
# If your line go through the edge of a brick, then the brick is not considered as crossed.
# You need to find out how to draw the line to cross the least bricks and return the number of crossed bricks.
#
# You cannot draw a line just along one of the two vertical edges of the wall,
# in which case the line will obviously cross no bricks.
#
# Example:
# Input: 
# [[1,2,2,1],
#  [3,1,2],
#  [1,3,2],
#  [2,4],
#  [3,1,2],
#  [1,3,1,1]]
# Output: 2
#
# Note:
# The width sum of bricks in different rows are the same and won't exceed INT_MAX.
# The number of bricks in each row is in range [1,10,000].
# The height of wall is in range [1,10,000].
# Total number of bricks of the wall won't exceed 20,000.

class Solution(object):
    def leastBricks(self, wall):
        """
        :type wall: List[List[int]]
        :rtype: int
        """
        widths = collections.defaultdict(int)
        result = len(wall)
        for row in wall:
            width = 0
            for i in xrange(len(row)-1):
                width += row[i]
                widths[width] += 1
                result = min(result, len(wall) - widths[width]);
        return result