LeetCode——复制图

http://articles.leetcode.com/2012/05/clone-graph-part-i.html

BFS方法遍历图。为了防止重复遍历，使用hashmap，key为原图的node，value为新图的node。

我没有看原文的算法实现先是自己想了然后写上去。这样的过程中我碰到了一些问题。比如说什么时候创建新的节点？是在while的开始，还是在遍历相邻节点的时候？这点很重要。

实现：

package com.song.algorithm;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.List;
import java.util.Queue;

public class Image {
	public static class Node {
		public int data;
		public List<Node> neighbors;
	}

	public Node head;

	public static Image copy(Image image) {
		Queue<Node> queue = new LinkedList<Node>();
		if (image.head == null) {
			return null;
		}
		Image cp = new Image();
		HashMap<Node, Node> map = new HashMap<Node, Node>();
		Node newHead = new Node();
		newHead.data = image.head.data;
		cp.head = newHead;
		map.put(image.head, newHead);
		queue.add(image.head);
		while (!queue.isEmpty()) {
			Node node = queue.remove();
			Node cnode = map.get(node);
			if (node.neighbors != null) {
				List<Node> cnodeNeighbors = new ArrayList<Node>();
				cnode.neighbors = cnodeNeighbors;
				for (Node n : node.neighbors) {
					if (map.get(n) == null) {
						Node newNode = new Node();
						newNode.data = n.data;
						map.put(n, newNode);
						cnodeNeighbors.add(newNode);
						queue.add(n);
					} else {
						cnodeNeighbors.add(map.get(n));
					}
				}
			}
		}
		return cp;
	}

	public static void iterate(Image image) {
		HashSet<Node> set = new HashSet<Node>();
		set.add(image.head);
		Queue<Node> queue = new LinkedList<Node>();
		queue.add(image.head);
		while (!queue.isEmpty()) {
			Node node = queue.remove();
			System.out.print(node.data + " ");
			if (node.neighbors != null) {
				for (Node n : node.neighbors) {
					if (!set.contains(n)) {
						queue.add(n);
						set.add(n);
					}
				}
			}
		}
	}

	public static void main(String[] args) {
		Image image = new Image();
		Node head = new Node();
		image.head = head;
		head.data = 1;
		ArrayList<Node> nei1 = new ArrayList<Node>();
		Node n2 = new Node();
		n2.data = 2;
		nei1.add(n2);
		head.neighbors = nei1;
		ArrayList<Node> nei2 = new ArrayList<Node>();
		Node n3 = new Node();
		n3.data = 3;
		nei2.add(n3);
		Node n4 = new Node();
		n4.data = 4;
		nei2.add(n4);
		n2.neighbors = nei2;
		ArrayList<Node> nei3 = new ArrayList<Node>();
		nei3.add(head);
		n4.neighbors = nei3;
		iterate(image);
		System.out.println("");
		Image image1 = copy(image);
		iterate(image1);
	}
}