问题
In C#, I have
class Pair{
int val1;
int val2;
}
I have a list of pairs coming from a source as:-
List<Pair> sList = new List<Pair>();
1 | 2
2 | 3
1 | 4
4 | 6
I need to convert it into the following type of structure:-
[1, [2, 3, 4, 6]]
[2, [3]]
[3, [2]]
[4, [1,6]]
[6, [4]]
What is the best way to go about this (without LINQ)?
回答1:
I would go with an ILookup<int, int>
, but you need to include the reverse associations as well:
var result = sList.Union(sList.Select(p => new Pair { val1 = p.val2, val2 = p.val1 }))
.ToLookup(p => p.val1, p => p.val2);
You can get a similar result without Linq using this:
var dict = new Dictionary<int, List<int>>();
foreach(var pair in sList)
{
if (!dict.ContainsKey(pair.val1))
{
dict[pair.val1] = new List<int>();
}
if (!dict.ContainsKey(pair.val2))
{
dict[pair.val2] = new List<int>();
}
dict[pair.val1].Add(pair.val2);
dict[pair.val2].Add(pair.val1);
}
Both of the methods above will produce an Adjacency List, however from your comments it sounds like what you want to do more like Connected Component Labeling
var groups = new List<HashSet<int>>();
foreach (var p in sList)
{
var merge = new List<HashSet<int>>();
foreach(var g in groups)
{
if (g.Contains(p.val1) || g.Contains(p.val2))
{
merge.Add(g);
}
}
if (merge.Count == 0)
{
var h = new HashSet<int>();
groups.Add(h);
merge.Add(h);
}
merge[0].Add(p.val1);
merge[0].Add(p.val2);
for(int i = 1; i < merge.Count; i ++)
{
foreach(int v in merge[i])
{
merge[0].Add(v);
}
groups.Remove(merge[i]);
}
}
When the input is
sList =
1 | 2
4 | 6
2 | 3
1 | 4
9 | 10
This will produce the output:
groups =
[ 1, 2, 3, 4, 6 ]
[ 9, 10 ]
It's then not too difficult to convert this to the format you want:
var dict = new Dictionary<int, List<int>>();
foreach(var g in groups)
{
foreach(var v in g)
{
var list = new List<int>(g);
list.Remove(g);
dict.Add(v, list)
}
}
Using the previous example:
dict =
1 | [ 2, 3, 4, 6 ]
2 | [ 1, 3, 4, 6 ]
3 | [ 1, 2, 4, 6 ]
4 | [ 1, 2, 3, 6 ]
6 | [ 1, 2, 3, 4 ]
9 | [ 9 ]
10 | [ 10 ]
回答2:
You can do this with LINQ's GroupBy
method, like this:
var adj = sList
.GroupBy(p => p.val1)
.ToDictionary(g => g.Key, g => g.Select(p => p.val2).ToList());
Note that this would not compute a transitive closure of your graph, i.e. only the direct links will be present.
In .NET 4 and up you can also use Tuple<int,int>
instead of making your own Pair
class.
来源:https://stackoverflow.com/questions/17355435/create-an-adjacency-list-type-structure-from-a-list-of-pairs