Depth-First Search (Recursive)

Visit a start vertex, then recurse into its first unvisited neighbour all the way down before backtracking. A visited set prevents revisiting, and neighbour insertion order fixes the visit sequence.

Algorithm

Basic Implementation

basic.swift

var adj: [Int: [Int]] = [:]
adj[1] = [2, 3]
adj[2] = [1, 4]
adj[3] = [1, 4]
adj[4] = [2, 3, 5]
adj[5] = [4, 6]
adj[6] = [5]
var visited: Set<Int> = []
var order: [Int] = []
func dfs(_ v: Int) {
	visited.insert(v)
	order.append(v)
	for nb in adj[v]! {
		if !visited.contains(nb) {
			dfs(nb)
		}
	}
}
dfs(1)
print(order)

DFS follows the first unvisited neighbour as far as it can, then unwinds and tries the next branch.

Step 1 - First branch

Starting from 1, insertion order sends DFS to 2, then 4, then 3.

Step 2 - Full DFS order

The final order is [1, 2, 4, 3, 5, 6], then calls unwind 6 -> 5 -> 4 -> 3 -> 2 -> 1.

Complexity

Time: O(V + E)
Space: O(V) recursion depth

Implementation notes

Swift: a recursive dfs function captures the global visited set and order array; print renders the array.
The replay shows the current vertex, the visited set, and the running visit order after each entry, matching the lesson spec.

recursive descent Follow one branch to its end, then unwind and try the next neighbour.