深度遍历

深度遍历，由于是图。所以我们需要用一个map维持clone过的节点，以及对应的新节点。遇到的时候直接赋值，避免再次陷入clone流程中。
递归最简单。

/**
 * 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;
        UndirectedGraphNode *nnode = new UndirectedGraphNode(node->label);
        visited[node] = nnode;
        for(int i = 0; i < node->neighbors.size(); i++) {
            UndirectedGraphNode *t = NULL;
            auto itr = visited.find(node->neighbors[i]);
            if(itr != visited.end()) t = itr->second;
            else t = cloneGraph(node->neighbors[i]);
            nnode->neighbors.push_back(t);
        }
        return nnode;
    }
    map<UndirectedGraphNode*, UndirectedGraphNode*> visited;
};