133 Clone Graph

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 #. First node is labeled as 0. Connect node 0 to both nodes 1 and 2. Second node is labeled as 1. Connect node 1 to node 2. 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
         / \
         \_/

Solution

/**
 * Definition for undirected graph.
 * struct UndirectedGraphNode {
 *     int label;
 *     vector<UndirectedGraphNode *> neighbors;
 *     UndirectedGraphNode(int x) : label(x) {};
 * };
 */
class Solution {
public:
    UndirectedGraphNode *cloneNode(UndirectedGraphNode *node, unordered_map<int, UndirectedGraphNode*>& cloned) {
        if ( cloned.find(node->label) != cloned.end() ) return cloned[node->label];
        UndirectedGraphNode *copy = new UndirectedGraphNode(node->label);
        cloned[node->label] = copy;
        for ( auto i: node->neighbors ) {
            auto *copy1 = cloneNode(i, cloned);
            copy->neighbors.push_back(copy1);
        }
        return copy;
    }
    UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
        if ( node == NULL ) return NULL;
        unordered_map<int, UndirectedGraphNode*> cloned;
        return cloneNode(node, cloned);
    }
};

Note

• Note to pass by reference via (unordered_set<int>& cloned), so that the change in cloned is recorded.
• Add the new node to the graph first, then connect to its neighbors. If I do it the other way around, the node connecting to itself will cause dead lock...