Two functions for collapsing links which are defined as python lists

links.py

from copy import copy

def collapse_links(links):
    """
    Example:
        >>> collapse_links([[0],[1,2],[2,3],[3,4],[4,5],[5]])
        [[0], [1, 2, 3, 4, 5]]
    """

    # Pointers
    left  = 0
    right = 1
    
    links = copy(links)

    for i in xrange(1,len(links)):
        if links[left][-1] == links[right][0]:
            if len(links[right]) > 1:
                # Add link
                links[left] += [links[right][-1]]
            # Mark for removal
            links[right] = None
        else:
            left = right

        right += 1

    return filter(None,links)

def collapse_links_with_mask(links,mask):
    """
    Example:
        >>> collapse_links_with_mask([[0],[1,2],[2,3],[3,4],[4,5],[5],[6],[7],[8]],[0,1,1,2,3,4,5,6,6])
        [[0], [1, 2, 3, 4, 5], [6], [7]]
    """

    # Pointers
    left  = 0
    right = 1
    
    links = copy(links)

    for i in xrange(1,len(links)):
        if mask[links[left][-1]] == mask[links[right][0]]:
            if len(links[right]) > 1:
                # Add link
                links[left] += [links[right][-1]]
            # Mark for removal
            links[right] = None
        else:
            left = right

        right += 1

    return filter(None,links)
    
collapse_links([[0],[1,2],[2,3],[3,4],[4,5],[5],[6],[7],[8]])
collapse_links_with_mask([[0],[1,2],[2,3],[3,4],[4,5],[5],[6],[7],[8]],[0,1,1,2,3,4,5,6,6])