133. Clone Graph

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
         / \
         \_/

链接： http://leetcode.com/problems/clone-graph/

题解：

拷贝图。图的遍历，主要就是DFS和BFS, 这道题考察基本功。需要注意的地点是如何建立visited数组，这道题因为lable unique，所以可以建立Map<Integer， UndirectedGraphNode>，假如lable有重复值，则建立Map的时候要使用Map<UndirectedGraphNode， UndirectedGraphNode>。 基本题目要多练习，这样拓展到难题以后才能借鉴思路。二刷的时候要注意recursive和iterative。

DFS:

Time Complexity - O(n)， Space Complexity - O(n)

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    HashMap<Integer, UndirectedGraphNode> visited = new HashMap<>();

    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null)
            return node;
        if(visited.containsKey(node.label))
            return visited.get(node.label);

        UndirectedGraphNode clone = new UndirectedGraphNode(node.label);
        visited.put(clone.label, clone);

        for(UndirectedGraphNode neighbor : node.neighbors)
            clone.neighbors.add(cloneGraph(neighbor));

        return clone;
    }
}

BFS:

Time Complexity - O(n)， Space Complexity - O(n)

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if(node == null)
            return node;

        Queue<UndirectedGraphNode> q = new LinkedList<>();
        q.offer(node);
        HashMap<Integer, UndirectedGraphNode> visited = new HashMap<>();
        visited.put(node.label, new UndirectedGraphNode(node.label));

        while(!q.isEmpty()) {
            UndirectedGraphNode newNode = q.poll();

            for(UndirectedGraphNode neighbor : newNode.neighbors) {
                if(!visited.containsKey(neighbor.label)) {
                    q.offer(neighbor);
                    visited.put(neighbor.label, new UndirectedGraphNode(neighbor.label));
                }
                visited.get(newNode.label).neighbors.add(visited.get(neighbor.label));
            }
        }

        return visited.get(node.label);
    }
}

二刷:

Java:

DFS:

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    Map<Integer, UndirectedGraphNode> map = new HashMap<>();

    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) return null;
        if (map.containsKey(node.label)) return map.get(node.label);
        UndirectedGraphNode newNode = new UndirectedGraphNode(node.label);
        map.put(newNode.label, newNode);

        for (UndirectedGraphNode neighbor : node.neighbors) {
            newNode.neighbors.add(cloneGraph(neighbor));
        }
        return newNode;
    }
}

BFS:

/**
 * Definition for undirected graph.
 * class UndirectedGraphNode {
 *     int label;
 *     List<UndirectedGraphNode> neighbors;
 *     UndirectedGraphNode(int x) { label = x; neighbors = new ArrayList<UndirectedGraphNode>(); }
 * };
 */
public class Solution {
    public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {
        if (node == null) return null;
        Queue<UndirectedGraphNode> q = new LinkedList<>();
        q.offer(node);
        Map<Integer, UndirectedGraphNode> visited = new HashMap<>();
        visited.put(node.label, new UndirectedGraphNode(node.label));

        while (!q.isEmpty()) {
            UndirectedGraphNode oldNode = q.poll();
            for (UndirectedGraphNode neighbor : oldNode.neighbors) {
                if (!visited.containsKey(neighbor.label)) {
                    q.offer(neighbor);
                    visited.put(neighbor.label, new UndirectedGraphNode(neighbor.label));
                }
                visited.get(oldNode.label).neighbors.add(visited.get(neighbor.label));
            }
        }
        return visited.get(node.label);
    }
}

Reference：

http://www.cnblogs.com/springfor/p/3874591.html

http://blog.csdn.net/linhuanmars/article/details/22715747

http://blog.csdn.net/fightforyourdream/article/details/17497883

http://www.programcreek.com/2012/12/leetcode-clone-graph-java/

https://leetcode.com/discuss/26988/depth-first-simple-java-solution

https://leetcode.com/discuss/44330/java-bfs-solution

https://leetcode.com/discuss/14969/simple-java-iterative-bfs-solution-with-hashmap-and-queue