import itertools

layer = iface.activeLayer()

feats = [ feat for feat in layer.getFeatures() ]

n = len(feats)

for i, j in itertools.combinations(range(n), 2):
    distance = feats[i].geometry().distance(feats[j].geometry())
    if distance < 50:
        print i, j, distance

0 3 0.0
0 5 6.32561007326e-08
1 2 0.0
1 4 2.77618203084e-08
2 3 0.0
4 6 3.69805894613e-10
5 6 5.00951291099e-08

0 3 0.0
0 7 6.32561007326e-08
1 2 0.0
1 6 2.77618203084e-08
2 3 0.0
4 5 6.09094222774e-08
4 8 3.69805894613e-10
5 6 22.0495367734
7 9 5.00951291099e-08
8 9 9.68909528121e-08

0 3 0.0
0 7 0.0
1 2 0.0
1 6 0.0
2 3 0.0
4 5 0.0
4 8 0.0
5 6 0.0
7 9 0.0
8 9 0.0

import itertools

layer = iface.activeLayer()

feats = [ feat for feat in layer.getFeatures() ]

n = len(feats)

indices = []

for i, j in itertools.combinations(range(n), 2):
    distance = feats[i].geometry().distance(feats[j].geometry())
    if distance < 50:
        indices.append([i,j])

selected_feats = []

selected_feats.append(feats[0])

del feats[0]

k=0

while len(feats) != 0:
    for i, feat in enumerate(feats):
        d = selected_feats[k].geometry().distance(feat.geometry())
        if d == 0:
            selected_feats.append(feat)
            k += 1
            del feats[i]

lines = [ feat.geometry().asPolyline() for feat in selected_feats ]

epsg = layer.crs().postgisSrid()

uri = "LineString?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer = QgsVectorLayer(uri,
                           '4testing_feats_ordered',
                           'memory')

prov = mem_layer.dataProvider()

feats = [ QgsFeature() for i in range(len(selected_feats)) ]

for i, feat in enumerate(feats):
    feat.setAttributes([i])
    feat.setGeometry(QgsGeometry.fromPolyline(lines[i]))

prov.addFeatures(feats)

QgsMapLayerRegistry.instance().addMapLayer(mem_layer)

registry = QgsMapLayerRegistry.instance()

layer = registry.mapLayersByName("4testing_feats_ordered")

lines = [ feat.geometry().asPolyline() for feat in layer[0].getFeatures() ]

sum = 0

for i in range(len(lines[0])-2):
    det =  (lines[0][i+1][0] - lines[0][i][0])*(lines[0][i+2][1] - lines[0][i][1])-(lines[0][i+1][1] - lines[0][i][1])*(lines[0][i+2][0] - lines[0][i][0])
    sum += det

if sum > 0:

    lines[0].reverse()

new_points = []

for point in lines[0]:
    new_points.append(point)

print new_points[-1].wellKnownText()

for i in range(1, len(lines), 1):
    if lines[i][-1] == new_points[-1]:
        lines[i].reverse()

    for point in lines[i]:
        new_points.append(point)

epsg = layer[0].crs().postgisSrid()

uri = "Point?crs=epsg:" + str(epsg) + "&field=id:integer""&index=yes"

mem_layer2 = QgsVectorLayer(uri,
                           'points',
                           'memory')

prov = mem_layer2.dataProvider()

feats = [ QgsFeature() for i in range(len(new_points)) ]

for i, feat in enumerate(feats):
    feat.setAttributes([i])
    feat.setGeometry(QgsGeometry.fromPoint(new_points[i]))

prov.addFeatures(feats)

QgsMapLayerRegistry.instance().addMapLayer(mem_layer2)