Validate a Directed Edge Addition

You are given a directed graph as a list of source-destination pairs and one proposed directed edge.
Return false if the exact proposed edge already exists or if adding it would create a directed cycle. Otherwise, return true.

Function
isValidEdgeAddition(edges: int[][], newEdge: int[]) → boolean

Examples
Example 1
edges = [[1,2],[2,3],[3,4]]
newEdge = [4,1]
return = false
Adding 4 -> 1 creates the cycle 1 -> 2 -> 3 -> 4 -> 1.
Example 2
edges = [[1,2],[2,3],[3,4]]
newEdge = [1,2]
return = false
The exact edge 1 -> 2 already exists.
Example 3
edges = [[1,2],[2,3]]
newEdge = [1,3]
return = true
The edge is new and does not create a directed cycle.

The trick: adding edge u -> v creates a cycle only if v can already reach u. So build an adjacency list, then run DFS or BFS from v and check whether you hit u. That's one traversal, O(V + E). Check the duplicate case first with a set of pairs, or just scan the edge list. The common pitfall is running full cycle detection with colors or Kahn's algorithm after inserting the edge. It works but it's more code and more bugs. The other pitfall is the self-loop. If u equals v, then v trivially reaches u, so return false. Also watch for nodes that appear only in newEdge and not in edges. Your map lookup should default to an empty list. Use an iterative traversal if you're worried about deep recursion. If you freeze in the live OA, StealthCoder can hand you this reachability approach so you can verify it against the three examples.

If this hits your live OA and you blank, StealthCoder solves it in seconds, invisible to the proctor.