How to Clone a Graph in Java
The challenge
Given a reference of a node in a connected undirected graph.
Return a deep copy (clone) of the graph. Effectively a graph copy.
Each node in the graph contains a val (int
) and a list (List[Node]
) of its neighbors.
Test case format:
For simplicity sake, each node’s value is the same as the node’s index (1indexed). For example, the first node with val = 1
, the second node with val = 2
, and so on. The graph is represented in the test case using an adjacency list.
Adjacency list is a collection of unordered lists used to represent a finite graph. Each list describes the set of neighbors of a node in the graph.
The given node will always be the first node with val = 1
. You must return the copy of the given node as a reference to the cloned graph. This will be a graph copy that we can use.
Example 1:
Input: adjList = [[2,4],[1,3],[2,4],[1,3]] Output: [[2,4],[1,3],[2,4],[1,3]] Explanation: There are 4 nodes in the graph. 1st node (val = 1)'s neighbors are 2nd node (val = 2) and 4th node (val = 4). 2nd node (val = 2)'s neighbors are 1st node (val = 1) and 3rd node (val = 3). 3rd node (val = 3)'s neighbors are 2nd node (val = 2) and 4th node (val = 4). 4th node (val = 4)'s neighbors are 1st node (val = 1) and 3rd node (val = 3).
Example 2:
Input: adjList = [[]] Output: [[]] Explanation: Note that the input contains one empty list. The graph consists of only one node with val = 1 and it does not have any neighbors.
Example 3:
Input: adjList = [] Output: [] Explanation: This an empty graph, it does not have any nodes.
Example 4:
Input: adjList = [[2],[1]] Output: [[2],[1]]
Constraints:
1 <= Node.val <= 100
Node.val
is unique for each node. Number of Nodes will not exceed 100.
 There is no repeated edges and no selfloops in the graph.
 The Graph is connected and all nodes can be visited starting from the given node.
The Definition for a Node


The solution in Java
We can solve this by means of a recursive traversal, as this is an undirected graph.
Start by visiting a node, move through it’s list of neighbours, call each neighbour recursively if they exist.
Implement a map
to check which nodes have already been called.

