def _checkBack(r, c, linko):
    """Checks if (r,c) lies on a backlink for a node."""
    finalNode = (r+c)//4

    if finalNode >= len(linko):
        return False
    
    backlinks = linko[finalNode][1]
    if len(backlinks) == 0:
        return False
    
    minBacklink = min(backlinks)

    return 2*minBacklink < 2*finalNode - c