133. Clone Graph

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Clone an undirected graph. Each node in the graph contains a label and a list of its neighbors.
OJ’s undirected graph serialization:
Nodes are labeled uniquely.
We use # as a separator for each node, and , as a separator for node label and each neighbor of the node.
As an example, consider the serialized graph {0,1,2#1,2#2,2}.
The graph has a total of three nodes, and therefore contains three parts as separated by #.
1. First node is labeled as 0. Connect node 0 to both nodes 1 and 2.
2. Second node is labeled as 1. Connect node 1 to node 2.
3. Third node is labeled as 2. Connect node 2 to node 2 (itself), thus forming a self-cycle.

Visually, the graph looks like the following:

       1      / \     /   \    0 --- 2         / \         \_/

题目大意

输入一个图的任意节点，克隆该图，并返回该克隆图。

解题思路

1. 创建克隆的graph，然后创建一个新的节点传入输入的节点的label，并将该新的节点作为克隆graph的起点。
2. 以输入的节点为起点，使用广度优先搜索，遍历所有的节点。
3. 为了避免重复访问点，使用map存储已访问的节点(key部分存储原始graph的节点，value部分存储克隆节点)，判断被访问节点是否被已被访问。如果没有被访问过，通过创建新的键值对，克隆的节点。
4. 同时，每次不管被访问节点是不是被访问过，都将其push入当前队列头部节点的neighbors。

算法复杂度

O(|V+E|)

代码实现

/** * Definition for undirected graph. * struct UndirectedGraphNode { *     int label; *     vector<UndirectedGraphNode *> neighbors; *     UndirectedGraphNode(int x) : label(x) {}; * }; */class Solution {public:  UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {    if (node == nullptr) { return nullptr; }    vector<UndirectedGraphNode*, UndirectedGraphNode*> visited;    vector<UndirectedGraphNode *>::iterator iter;    UndirectedGraphNode* graph = new UndirectedGraphNode(node->label);    visited[node] = graph;    queue<UndirectedGraphNode*> que;      que.push(node);    // BFS    while (!que.empty()) {      UndirectedGraphNode* u = que.front();      que.pop();      for (iter = u->neighbors.begin(); iter != u->neighbors.end(); iter++) {        // determine whether it has been visited        if (visited.find((*iter)) == visited.end()) {          visited[*iter] = new UndirectedGraphNode((*iter)->label);  // clone the label          que.push(*iter);        }        visited[u]->neighbors.push_back(visited[*iter]);  // clone the neighbors      }    }    return graph;  }};