AllOoneDataStructure.java

package com.dbc;

import java.util.HashMap;
import java.util.HashSet;
import java.util.Map;
import java.util.Set;

public class AllOoneDataStructure {
    class Node {
        Node prev;
        Node next;
        Set<String> keys;
        int count;

        public Node() {
            this("", 0);
        }

        public Node(String key, int count) {
            this.count = count;
            keys = new HashSet<String>();
            keys.add(key);
        }

        public Node insert(Node node) {  // 在 this 后插入 node
            node.prev = this;
            node.next = this.next;
            node.prev.next = node;
            node.next.prev = node;
            return node;
        }

        public void remove() {
            this.prev.next = this.next;
            this.next.prev = this.prev;
        }
    }

    Node root;
    Map<String, Node> nodes;

    public AllOoneDataStructure() {
        root = new Node();
        root.prev = root;
        root.next = root;  // 初始化链表哨兵，下面判断节点的 next 若为 root，则表示 next 为空（prev 同理）
        nodes = new HashMap<String, Node>();
    }

    public void inc(String key) {
        if (nodes.containsKey(key)) {
            Node cur = nodes.get(key);
            Node nxt = cur.next;
            if (nxt == root || nxt.count > cur.count + 1) {
                nodes.put(key, cur.insert(new Node(key, cur.count + 1)));
            } else {
                nxt.keys.add(key);
                nodes.put(key, nxt);
            }
            cur.keys.remove(key);
            if (cur.keys.isEmpty()) {
                cur.remove();
            }
        } else {  // key 不在链表中
            if (root.next == root || root.next.count > 1) {
                nodes.put(key, root.insert(new Node(key, 1)));
            } else {
                root.next.keys.add(key);
                nodes.put(key, root.next);
            }
        }
    }

    public void dec(String key) {
        Node cur = nodes.get(key);
        if (cur.count == 1) {  // key 仅出现一次，将其移出 nodes
            nodes.remove(key);
        } else {
            Node pre = cur.prev;
            if (pre == root || pre.count < cur.count - 1) {
                nodes.put(key, cur.prev.insert(new Node(key, cur.count - 1)));
            } else {
                pre.keys.add(key);
                nodes.put(key, pre);
            }
        }
        cur.keys.remove(key);
        if (cur.keys.isEmpty()) {
            cur.remove();
        }
    }

    public String getMaxKey() {
        return root.prev != null ? root.prev.keys.iterator().next() : "";
    }

    public String getMinKey() {
        return root.next != null ? root.next.keys.iterator().next() : "";
    }
}