133. Clone Graph 图的复制，运用了BFS

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 

#

.
First node is labeled as 

0

. Connect node 

0

 to
both nodes 

1

 and 

2

.
Second node is labeled as 

1

. Connect node 

1

 to
node 

2

.
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
/ \
\_/

1.我的解答，看不懂题目，看了别人的代码
反正就是返回给定node的邻居节点。用了BFS的queue
/**
* 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 == NULL) return NULL;
//1.BFS遍历每个node,并为每个node创建数组[label,UndirectedGraphNode *]
map<int, UndirectedGraphNode *>mp;
queue<UndirectedGraphNode *>q;
q.push(node);
while(!q.empty()){
UndirectedGraphNode *temp = q.front();
q.pop();
if(mp.find(temp->label) == mp.end()){ //意味着该点未被访问过
UndirectedGraphNode *n = new UndirectedGraphNode(temp->label);
mp[temp->label] = n;
for(int i = 0; i < (temp->neighbors.size()); i++){
q.push(temp->neighbors[i]);
}
}
}

//2.BFS依然遍历每个node,然后对数组中的每个点都加入邻居点
q.push(node);
while(!q.empty()){
UndirectedGraphNode *temp = q.front();
q.pop();
UndirectedGraphNode *target = mp[temp->label];
if(target->neighbors.empty() && (!temp->neighbors.empty())){
for(int i = 0; i < (temp->neighbors.size()); i++){
target->neighbors.push_back(mp[temp->neighbors[i]->label]);
q.push(temp->neighbors[i]);
}
}
}
return mp[node->label];
}
};

2.别人的解释和代码

This problem is an application of graph traversal, which has two systematic methods: Bread-First Search (BFS) and Depth-First Search (DFS). In the following, I am going to assume that you are familiar with them and
just focus on what I think is the most tricky part of this problem, that is, what else is needed beyond graph traversal to clone a graph?

In order to clone a graph, you need to have a copy of each node in the original graph. Well, you may not have too many ideas about it. Let's do an example.

Suppose we are given a graph 

{0, 1 # 1, 0}

. We know that the graph has two nodes 

0

 and 

1

 and
they are connected to each other.

We now start from 

0

. We make a copy of 

0

.
Then we check 

0

's neighbors and we see 

1

.
We make a copy of 

1

 and we add the copy to the neighbors of the copy of 

0

.
Now the cloned graph is 

{0 (copy), 1 (copy)}

. Then we visit 

1

.
We make a copy of 

1

... well, wait, why do we make another copy of it? We already have
one! Note that if you make a new copy of the node, these copies are not the same and the graph structure will be wrong! This is just what I mean by "the most tricky part of this problem". In fact, we need to maintain a mapping from each
node to its copy. If the node has an existed copy, we simply use it. So in the above example, the remaining process is that we visit the copy of 

1

 and
add the copy of 

0

 to its neighbors and the cloned graph is eventually 

{0
(copy), 1 (copy) # 1 (copy), 0 (copy)}

.

Note that the above process uses BFS. Of course, you can use DFS. The key is the node-copy mapping, anyway.
（1）BFS

class Solution {
public:
UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
if (!node) return NULL;
UndirectedGraphNode* copy = new UndirectedGraphNode(node -> label);
mp[node] = copy;
queue<UndirectedGraphNode*> toVisit;
toVisit.push(node);
while (!toVisit.empty()) {
UndirectedGraphNode* cur = toVisit.front();
toVisit.pop();
for (UndirectedGraphNode* neigh : cur -> neighbors) {
if (mp.find(neigh) == mp.end()) {
UndirectedGraphNode* neigh_copy = new UndirectedGraphNode(neigh -> label);
mp[neigh] = neigh_copy;
toVisit.push(neigh);
}
mp[cur] -> neighbors.push_back(mp[neigh]);
}
}
return copy;
}
private:
unordered_map<UndirectedGraphNode*, UndirectedGraphNode*> mp;
};

（2）DFS
class Solution {
public:
UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
if (!node) return NULL;
if (mp.find(node) == mp.end()) {
mp[node] = new UndirectedGraphNode(node -> label);
for (UndirectedGraphNode* neigh : node -> neighbors)
mp[node] -> neighbors.push_back(cloneGraph(neigh));
}
return mp[node];
}
private:
unordered_map<UndirectedGraphNode*, UndirectedGraphNode*> mp;
};