133 Clone Graph [Leetcode]

題目內容：

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
     / \
     \_/

解題思路：
一邊遍歷圖一邊複製圖。爲了防止重複遍歷，使用一個hash表來記錄已經創建了的節點。這裏使用了DFS來遍歷圖並進行復制，使用遞歸實現。

代碼如下，運行時間72ms：

/**
 * Definition for undirected graph.
 * struct UndirectedGraphNode {
 *     int label;
 *     vector<UndirectedGraphNode *> neighbors;
 *     UndirectedGraphNode(int x) : label(x) {};
 * };
 */
class Solution {
private:
    unordered_map<int, UndirectedGraphNode *> graphNodes;
public:
    UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
        if(node == NULL)
            return NULL;
        if(graphNodes.find(node->label) == graphNodes.end()) {
            UndirectedGraphNode *cnode = new UndirectedGraphNode(node->label);
            graphNodes[node->label] = cnode;

            int size(node->neighbors.size());
            if(size > 0) {
                cnode->neighbors.resize(size);
                for(int i = 0; i < size; ++i) {
                    cnode->neighbors[i] = cloneGraph(node->neighbors[i]);
                }
            }
            return cnode;
        }
        else
            return graphNodes[node->label];
    }
};