[Leetcode] 133. Clone Graph 解题报告

题目：

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
         / \
         \_/

思路：

解决图的问题的最大神器依然是DFS和BFS。由于图有可能会形成回路，所以我们建立一个哈希表来存储已经clone后的节点。在本题目中，我们采用DFS的思路，代码十分简练和易懂。

代码：

/**
 * Definition for undirected graph.
 * struct UndirectedGraphNode {
 *     int label;
 *     vector neighbors;
 *     UndirectedGraphNode(int x) : label(x) {};
 * };
 */
class Solution {
public:
    UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
        if(node == NULL) {
            return NULL;
        }
        if(hash.count(node)) {
            return hash[node];
        }
        hash[node] = new UndirectedGraphNode(node->label);
        for(auto val : node->neighbors) {
            hash[node]->neighbors.push_back(cloneGraph(val));
        }
        return hash[node];
    }
private:
    unordered_map hash;
};