LintCode 137. Clone Graph

1. 需要掌握图的表示方法，图分为两种常见表示：邻接表，邻接矩阵。主要有一个neighbors的vector来存所有邻居的信息
2. 遇到图的题，要遍历图，一般选择BFS，因为宽度优先遍历可以用队列实现，不会发生栈溢出的情况
3. 遇到图的题，要遍历图，需要一个Hashmap，因为图可能有环，尤其对于无向图，为了避免二次访问，Hashmap非常重要
4. 需要记忆，c++中Hashmap用unordered_map来表示
5. 需要记忆，mapping.find(x) == mapping.end()来判断是否已存储了某值
6. 克隆图，便是遍历一遍，生成点和边，注意可以一遍生成没必要遍历两遍。每次while都是生成neighbors的点和front的之间的边。

/**
 * Definition for undirected graph.
 * struct UndirectedGraphNode {
 *     int label;
 *     vector<UndirectedGraphNode *> neighbors;
 *     UndirectedGraphNode(int x) : label(x) {};
 * };
 */


class Solution {
public:
    /*
     * @param node: A undirected graph node
     * @return: A undirected graph node
     */
    UndirectedGraphNode* cloneGraph(UndirectedGraphNode* node) {
        if (node == NULL) {
            return NULL;
        }
        
        queue<UndirectedGraphNode *> q;
        q.push(node);
        unordered_map<UndirectedGraphNode *, UndirectedGraphNode *> mapping;
        mapping[node] = new UndirectedGraphNode(node->label);
        
        while (!q.empty()) {
            UndirectedGraphNode * head = q.front();
            q.pop();
            for (UndirectedGraphNode * neighbor: head->neighbors) {
                if (mapping.find(neighbor) == mapping.end()) {
                    mapping[neighbor] = new UndirectedGraphNode(neighbor->label);
                    q.push(neighbor);
                }
                mapping[head]->neighbors.push_back(mapping[neighbor]);
            }
        }
        return mapping[node];
    }
};