Clone Graph -- leetcode

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
         /          \_/

给出一个无向连通图，要求复制

基本思路：

对图的遍历，采取广度优先或者深度优先。

遍历时，需要记住已访问的结点。避免重复访问。这功能可以和下面的map重用。

另外需要一个map， 映射，当前节点，和其对应的复制节点。

访问每一个节点时，需要复制其邻接边。对题目来讲，就是复制其 neighbours数组。

当边所引用的节点不存在时，需要创建此结点。

以下深度优先实现方式。在leetcode上实际执行时间为 72ms。

/**
 * 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) return node;
        stack<UndirectedGraphNode *> s;
        unordered_map<UndirectedGraphNode*, UndirectedGraphNode*> m;
        s.push(node);
        auto root = new UndirectedGraphNode(node->label);
        m[node] = root;
        while (!s.empty()) {
            node = s.top();
            s.pop();
            auto node_copy = m[node];
            for (auto neighbor: node->neighbors) {
                auto &copy = m[neighbor];
                if (!copy) {
                    s.push(neighbor);
                    copy = new UndirectedGraphNode(neighbor->label);
                }
                node_copy->neighbors.push_back(copy);
            }
        }
        return root;
    }
};

广度优先实际方式。在leetcode上执行时间为76ms。

即将上面算法的stack换成了queue。

class Solution {
public:
    UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
        if (!node) return node;
        queue<UndirectedGraphNode *> q;
        unordered_map<UndirectedGraphNode *, UndirectedGraphNode *> m;
        q.push(node);
        auto root_copy = new UndirectedGraphNode(node->label);
        m[node] = root_copy;
        while (!q.empty()) {
            node = q.front();
            q.pop();
            auto node_copy = m[node];
            for (auto neighbor: node->neighbors) {
                auto &copy = m[neighbor];
                if (!copy) {
                    q.push(neighbor);
                    copy = new UndirectedGraphNode(neighbor->label);
                }
                node_copy->neighbors.push_back(copy);
            }
        }
        return root_copy;
    }
};