利用双向链表实现LRU算法（leetcode146）

class LRUCache {

    private int capacity;
    private Map<Integer, Node> cache;
    private Node head;
    private Node tail;

    public LRUCache(int capacity) {
        this.capacity = capacity;
        this.cache = new HashMap<>();
    }

    public int get(int key) {
        Node res = cache.get(key);
        if (res == null) {
            return -1;
        }
        put(key, res.val);
        return res.val;
    }

    public void put(int key, int value) {
        Node newNode = new Node(key, value);
        if (cache.containsKey(key)) {
            removeNode(cache.get(key));
            addHead(newNode);
            cache.put(key, newNode);
        } else {
            if (cache.size() == capacity) {
                Node x = removeTail();
                cache.remove(x.key);
                addHead(newNode);
                cache.put(key, newNode);
            } else {
                addHead(newNode);
                cache.put(key, newNode);
            }
        }
    }

    public Node removeTail() {
        Node temp = tail;
        Node pre = tail.pre;
        if (pre == null) {
            //0号节点
            head = null;
            tail = head;
            return temp;
        }
        pre.next = null;
        tail = pre;
        return temp;
    }

    public void addHead(Node x) {
        if (head == null) {
            head = x;
            tail = head;
            return;
        }
        x.next = head;
        head.pre = x;
        head = x;
    }

    public void removeNode(Node x) {
        Node pre = x.pre;
        Node next = x.next;
        if (pre == null) {
            if (next == null) {
                head = null;
                tail = head;
                return;
            }
            next.pre = null;
            head = next;
            return;
        }
        if (next == null) {
            pre.next = null;
            tail = pre;
            return;
        }
        pre.next = next;
        next.pre = pre;

    }

    class Node {
        Node pre;
        Node next;
        int key;
        int val;

        public Node(int key, int val) {
            this.key = key;
            this.val = val;
        }
    }

}
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101

题目：https://leetcode-cn.com/problems/lru-cache/