Leetcode刷题系列（十七）Graph相关

　　本篇给出几道关于图的问题。但是没有深度搜索和广度搜索的题型，仅是与图结构相关的。

Clone Graph

　　这道题为深度克隆一张图。我们分点和点的对应关系分别进行克隆。

public UndirectedGraphNode cloneGraph(UndirectedGraphNode node) {    if (node == null) {        return node;    }    // use bfs algorithm to traverse the graph and get all nodes.    ArrayList<UndirectedGraphNode> nodes = getNodes(node);    // copy nodes, store the old->new mapping information in a hash map    HashMap<UndirectedGraphNode, UndirectedGraphNode> mapping = new HashMap<>();    for (UndirectedGraphNode n : nodes) {        mapping.put(n, new UndirectedGraphNode(n.label));    }    // copy neighbors(edges)    for (UndirectedGraphNode n : nodes) {        UndirectedGraphNode newNode = mapping.get(n);        for (UndirectedGraphNode neighbor : n.neighbors) {            UndirectedGraphNode newNeighbor = mapping.get(neighbor);            newNode.neighbors.add(newNeighbor);        }    }    return mapping.get(node);}private ArrayList<UndirectedGraphNode> getNodes(UndirectedGraphNode node) {    Queue<UndirectedGraphNode> queue = new LinkedList<UndirectedGraphNode>();    HashSet<UndirectedGraphNode> set = new HashSet<>();    queue.offer(node);    set.add(node);    while (!queue.isEmpty()) {        UndirectedGraphNode head = queue.poll();        for (UndirectedGraphNode neighbor : head.neighbors) {            if(!set.contains(neighbor)){                set.add(neighbor);                queue.offer(neighbor);            }        }    }    return new ArrayList<UndirectedGraphNode>(set);}

Copy List with Random Pointer

　　这道题是深度克隆一个链表，链表上的节点带随机指针。与上道题相似，先克隆节点再克隆关系。

public RandomListNode copyRandomList(RandomListNode head) {    if (head == null) {        return null;    }    HashMap<RandomListNode, RandomListNode> map = new HashMap<RandomListNode, RandomListNode>();    RandomListNode dummy = new RandomListNode(0);    RandomListNode pre = dummy, newNode;    while (head != null) {        if (map.containsKey(head)) {            newNode = map.get(head);        } else {            newNode = new RandomListNode(head.label);            map.put(head, newNode);        }        pre.next = newNode;        if (head.random != null) {            if (map.containsKey(head.random)) {                newNode.random = map.get(head.random);            } else {                newNode.random = new RandomListNode(head.random.label);                map.put(head.random, newNode.random);            }        }        pre = newNode;        head = head.next;    }    return dummy.next;}

Topological Sorting

　　这道题是有序图的拓扑排序的标准算法，理解拓扑排序即可理解下面代码的过程。

public ArrayList<DirectedGraphNode> topSort(ArrayList<DirectedGraphNode> graph) {    // write your code here    ArrayList<DirectedGraphNode> result = new ArrayList<DirectedGraphNode>();    HashMap<DirectedGraphNode, Integer> map = new HashMap();    for (DirectedGraphNode node : graph) {        for (DirectedGraphNode neighbor : node.neighbors) {            if (map.containsKey(neighbor)) {                map.put(neighbor, map.get(neighbor) + 1);            } else {                map.put(neighbor, 1);             }        }    }    Queue<DirectedGraphNode> q = new LinkedList<DirectedGraphNode>();    for (DirectedGraphNode node : graph) {        if (!map.containsKey(node)) {            q.offer(node);            result.add(node);        }    }    while (!q.isEmpty()) {        DirectedGraphNode node = q.poll();        for (DirectedGraphNode n : node.neighbors) {            map.put(n, map.get(n) - 1);            if (map.get(n) == 0) {                result.add(n);                q.offer(n);            }        }    }    return result;}