Problem

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 value (int) and a list (List[Node]) of its neighbors.

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

Test case format:

For simplicity, 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.

An 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.

https://leetcode.com/problems/clone-graph/

Example 1:

case1

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:

case2

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.

Constraints:

  • The number of nodes in the graph is in the range [0, 100].
  • 1 <= Node.val <= 100
  • Node.val is unique for each node.
  • There are no repeated edges and no self-loops in the graph.
  • The Graph is connected and all nodes can be visited starting from the given node.

Test Cases

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"""
# 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 []
"""

from typing import Optional
class Solution:
    def cloneGraph(self, node: Optional['Node']) -> Optional['Node']:
solution_test.py
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from typing import Optional

import pytest

from node import Node
from solution import Solution


def build_graph(adjacencies: list[list[int]]) -> Optional['Node']:
    n = len(adjacencies)
    nodes = [Node(i + 1) for i in range(n)]
    for node, neighbors in zip(nodes, adjacencies):
        node.neighbors[:] = [nodes[j - 1] for j in neighbors]

    return nodes[0] if n > 0 else None


def get_adjacencies(node: Optional['Node']) -> list[list[int]]:
    if node is None:
        return []

    nodes: dict[int, Node] = {}
    visit_stack: list[Node] = []
    if node is not None:
        visit_stack.append(node)

    # depth-first by stack. To use breadth-first, try queue.
    while visit_stack:
        n1 = visit_stack.pop()
        if n1.val in nodes:
            continue
        nodes[n1.val] = n1
        visit_stack.extend(filter(lambda n2: n2.val not in nodes, n1.neighbors))

    return [[n2.val for n2 in nodes[i + 1].neighbors] for i in range(len(nodes))]


@pytest.mark.parametrize('adjacencies', [
    ([[2,4],[1,3],[2,4],[1,3]]),
    ([[]]),
    ([]),
])
class Test:
    def test_solution(self, adjacencies):
        node = build_graph(adjacencies)
        sol = Solution()
        res = sol.cloneGraph(node)

        if node is not None:
            assert res is not node

        actual = get_adjacencies(res)
        for neighbors in actual:
            neighbors.sort()

        assert actual == adjacencies

Thoughts

写测试方法的时候相当于已经有解题的方法了。

核心就是遍历一遍 graph，确保找到所有的节点。图的遍历可以有深度优先或广度优先。

避免使用递归，可以用栈或队列缓存需要访问的节点，用循环替代递归。如果用栈（后进先出），相当于深度优先。如果用队列（先进先出），相当于广度优先。

可以用哈希表记录访问过的节点，key 是节点的 val，值是节点对象。也可以用数组，以 val 作为数组下标，只是需要动态扩大数组长度。

拿到所有节点之后，生成一组对应的新节点，并把每个新节点的 neighbors 设置好即可。

假设一共有 n 个节点，m 条边，则遍历所有节点和 clone 的时间复杂度都是 O(n + m)，空间复杂度为 O(n + m)。

Code

solution.py
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"""
# 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 []
"""

from typing import Optional

from node import Node


class Solution:
    def cloneGraph(self, node: Optional['Node']) -> Optional['Node']:
        if node is None:
            return None

        nodes: dict[int, 'Node'] = {}
        stack: list['Node'] = [node]

        # depth-first via stack.
        while stack:
            n1 = stack.pop()
            if n1.val in nodes:
                continue

            nodes[n1.val] = n1
            stack.extend(filter(lambda n2: n2.val not in nodes, n1.neighbors))

        n = len(nodes)
        cloned_nodes = [Node(i + 1) for i in range(n)]
        for cloned_n1 in cloned_nodes:
            n1 = nodes[cloned_n1.val]
            cloned_n1.neighbors[:] = [cloned_nodes[n2.val - 1] for n2 in n1.neighbors]

        return cloned_nodes[0]
medium
许可协议

本文除了题目描述（Problem）部分之外，其他内容由 Calf 原创，采用 CC BY-NC-SA 4.0 许可协议，转载请注明出处。

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