Graphs

Build a Graph as an Adjacency List

Represent an undirected graph as a per-vertex list of neighbours. For every edge (u, v), append v to adj[u] and u to adj[v]. Neighbour lists keep insertion order so the graph is a stable, deterministic fixture for the search lessons.

Algorithm

Basic Implementation

basic.swift 
let edges = [[1, 2], [1, 3], [2, 4], [3, 4], [4, 5], [5, 6]]
var adj: [Int: [Int]] = [:]
for e in edges {
	adj[e[0], default: []].append(e[1])
	adj[e[1], default: []].append(e[0])
}
var parts: [String] = []
for v in adj.keys.sorted() {
	let nbrs = adj[v]!.map { String($0) }.joined(separator: ", ")
	parts.append("\(v): [\(nbrs)]")
}
print("{" + parts.joined(separator: ", ") + "}")

The graph fixture is pinned once, then the adjacency list writes each undirected edge in both directions.

Step 1 - Pinned graph fixture

The six vertices and six undirected edges are the shared fixture for BFS and DFS.

Graph with edges (1,2), (1,3), (2,4), (3,4), (4,5), (5,6).123456

Step 2 - Final adjacency list

Each row lists neighbours in the same insertion order used by the lesson.

Adjacency list after all six undirected edges are inserted.vertexneighbours1[2, 3]2[1, 4]3[1, 4]4[2, 3, 5]5[4, 6]6[5]

Complexity

  • Build: O(V + E)
  • Space: O(V + E)

Implementation notes

  • Swift: a [Int: [Int]] dictionary stores neighbours via subscript-with-default; keys are sorted before printing because dictionary order is unspecified.
  • The replay shows the adjacency list after each edge is added, matching the lesson spec.
adjacency list Each edge adds two directed entries, one in each direction.