Answers for "topological sort using ~operator"

Topological sort

L ← Empty list that will contain the sorted nodes
while exists nodes without a permanent mark do
    select an unmarked node n
    visit(n)

function visit(node n)
    if n has a permanent mark then
        return
    if n has a temporary mark then
        stop   (not a DAG)

    mark n with a temporary mark

    for each node m with an edge from n to m do
        visit(m)

    remove temporary mark from n
    mark n with a permanent mark
    add n to head of L

topological sort

def topological_sort():
    for each node:
        if visited[node] is False:
            dfs(node)
def dfs(node):
    visited[node] = True
    for nei in neighbours[node]:
        dfs(node)
    ret.insert_at_the _front(node)