LeetCode 133 Clone Graph - Medium

133. Clone Graph — Medium

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Problem

133. Clone Graph

Given a reference of a node in a connected undirected graph.

Return a deep copy (clone) of the graph.

Each node in the graph contains a val (int) and a list (List[Node]) of its neighbors.

class Node { public int val; public List neighbors; }

Test case format:

For simplicity sake, each node's value is the same as the node's index (1-indexed). 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.

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 self-loops in the graph. The Graph is connected and all nodes can be visited starting from the given nod

Solution

"""
# Definition for a Node.
class Node:
    def __init__(self, val = 0, neighbors = None):
        self.val = val
        self.neighbors = neighbors if neighbors is not None else []
"""
 
class Solution:
    def cloneGraph(self, node):
        root = node
        if not node:
            return 
        ### BFS to transverse the graph and get all nodes:
        nodes = self.getNodes(root)
        ### mapping old with new nodes:
        ### mapping key old node, value: new Node
        mapping = {}
        for node in nodes:
            ### at the same time creating, the new node using Node() class, using the old node's val first.
            mapping[node] = Node(node.val)
        
        ### mapping, adding neighbors:
        for node in nodes:
            #get new node from mapped hashmap
            new_node = mapping[node]
            #get neighbor list from old node
            for neighbor in node.neighbors: #neignbor are nodes too, so we can find them from the mapping
                #find this neighbor in the mapped hashmap, where we used the old node key and get new node value. so mapping[old key] = new node
                new_neighbor = mapping[neighbor]
                #add this neighbor to the new_node's atrribute
                new_node.neighbors.append(new_neighbor)
        
        return mapping[root]
        
    def getNodes(self, node):
        queue = collections.deque([node])
        res = set()
        res.add(node)
        while queue:
            head = queue.popleft()
            for neighbor in head.neighbors:
                if neighbor not in res:
                    res.add(neighbor)
                    queue.append(neighbor)
        return res