Question

I have a collection:

List<VPair<Item, List<Item>> dependencyHierarchy;

The first item in pair is some object (item) and the second one is a collection of the same type objects that the first one depends on. I want to get a List<Item> in order of dependency, so there are not items that depend on the first element and so on (no cycled dependency!).

Input:

Item4 depends on Item3 and Item5
Item3 depends on Item1
Item1 does not depend on any one
Item2 depends on Item4 
Item5 does not depend on any one 

Result:

Item1
Item5
Item3
Item4
Item2

Thank you.

SOLUTION:

Topological Sorting (thanks to Loïc Février for idea)

and

example on C#, example on Java (thanks to xcud for great examples)

Answer 1

Perfect example to use a topological sort:

http://en.wikipedia.org/wiki/Topological_sorting

It will give you exactly what you need.

You can either use Kahn’s algorithm:

L ← Empty list that will contain the sorted elements
S ← Set of all nodes with no incoming edge

while S is not empty do
    remove a node n from S
    add n to L
    for each node m with an edge e from n to m do
        remove edge e from the graph
        if m has no other incoming edges then
            insert m into S

if graph has edges then
    return error   (graph has at least one cycle)
else 
    return L   (a topologically sorted order)

…or you can use Depth-first search:

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