Push Dominoes — Python

# Time:  O(n)
# Space: O(n)


class Solution(object):
    def pushDominoes(self, dominoes):
        """
        :type dominoes: str
        :rtype: str
        """
        force = [0]*len(dominoes)

        f = 0
        for i in xrange(len(dominoes)):
            if dominoes[i] == 'R':
                f = len(dominoes)
            elif dominoes[i] == 'L':
                f = 0
            else:
                f = max(f-1, 0)
            force[i] += f

        f = 0
        for i in reversed(xrange(len(dominoes))):
            if dominoes[i] == 'L':
                f = len(dominoes)
            elif dominoes[i] == 'R':
                f = 0
            else:
                f = max(f-1, 0)
            force[i] -= f

        return "".join('.' if f == 0 else 'R' if f > 0 else 'L'
                       for f in force)