# Source code for tryalgo.topological_order

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
"""\
Topological order
jill-jênn vie et christoph dürr - 2014-2019
"""
# snip{ topological_order_dfs
[docs]
def topological_order_dfs(graph):
"""Topological sorting by depth first search
:param graph: directed graph in listlist format, cannot be listdict
:returns: list of vertices in order
:complexity: `O(|V|+|E|)`
"""
n = len(graph)
order = []
times_seen = [-1] * n
for start in range(n):
if times_seen[start] == -1:
times_seen[start] = 0
to_visit = [start]
while to_visit:
node = to_visit[-1]
children = graph[node]
if times_seen[node] == len(children):
to_visit.pop()
order.append(node)
else:
child = children[times_seen[node]]
times_seen[node] += 1
if times_seen[child] == -1:
times_seen[child] = 0
to_visit.append(child)
return order[::-1]
# snip}
# snip{
[docs]
def topological_order(graph):
"""Topological sorting by maintaining indegree
:param graph: directed graph in listlist format, cannot be listdict
:returns: list of vertices in order
:complexity: `O(|V|+|E|)`
"""
V = range(len(graph))
indeg = [0 for _ in V]
for node in V: # compute indegree
for neighbor in graph[node]:
indeg[neighbor] += 1
Q = [node for node in V if indeg[node] == 0]
order = []
while Q:
node = Q.pop() # node without incoming arrows
order.append(node)
for neighbor in graph[node]:
indeg[neighbor] -= 1
if indeg[neighbor] == 0:
Q.append(neighbor)
return order
# snip}