Thursday, November 20, 2014

[LeetCode] 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 #.
  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

和Copy List with Random Pointer那题的思路一样。用一个hash table记录原图节点和复制图节点间的对应关系,以防止重复建立节点。和那题的不同在于遍历原图相对比linked list的情况复杂一点。可以用BFS或DFS来遍历原图。而hash table本身除了记录对应关系外,还有记录原图中每个节点是否已经被visit的功能。

BFS遍历:

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class Solution {
public:
    UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
        if(!node) return NULL;
        UndirectedGraphNode *p1 = node;
        UndirectedGraphNode *p2 = new UndirectedGraphNode(node->label);
        unordered_map<UndirectedGraphNode*, UndirectedGraphNode*> ht;
        queue<UndirectedGraphNode*> q;        
        q.push(node);
        ht[node] = p2;
        
        while(!q.empty()) {
            p1 = q.front();
            p2 = ht[p1];
            q.pop();
            for(int i=0; i<p1->neighbors.size(); i++) {
                UndirectedGraphNode *nb = p1->neighbors[i];
                if(ht.count(nb)) {
                    p2->neighbors.push_back(ht[nb]);
                }
                else {
                    UndirectedGraphNode *temp = new UndirectedGraphNode(nb->label);
                    p2->neighbors.push_back(temp);
                    ht[nb] = temp;
                    q.push(nb);
                }
            }
        }
        
        return ht[node];
    }
};


DFS遍历:

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class Solution {
public:
    UndirectedGraphNode *cloneGraph(UndirectedGraphNode *node) {
        if(!node) return NULL;
        unordered_map<UndirectedGraphNode*, UndirectedGraphNode*> ht;
        stack<UndirectedGraphNode*> s;        
        s.push(node);
        ht[node] = new UndirectedGraphNode(node->label);
        
        while(!s.empty()) {
            UndirectedGraphNode *p1 = s.top(), *p2 = ht[p1];
            s.pop();
            
            for(int i=0; i<p1->neighbors.size(); i++) {
                UndirectedGraphNode *nb = p1->neighbors[i];
                if(ht.count(nb)) {
                    p2->neighbors.push_back(ht[nb]);
                }
                else {
                    UndirectedGraphNode *temp = new UndirectedGraphNode(nb->label);
                    p2->neighbors.push_back(temp);
                    ht[nb] = temp;
                    s.push(nb);
                }
            }
        }
        
        return ht[node];
    }
};

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