Leetcode/各大家--146. LRU Cache(HashMap + Doubly LinkedList)

146. LRU Cache(HashMap + Doubly LinkedList)
hard
https://leetcode.com/problems/lru-cache/

Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and put.

get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.
put(key, value) - Set or insert the value if the key is not already present. When the cache reached its capacity, it should invalidate the least recently used item before inserting a new item.

Follow up:
Could you do both operations in O(1) time complexity?

Example:

LRUCache cache = new LRUCache( 2 /* capacity */ );

cache.put(1, 1);
cache.put(2, 2);
cache.get(1);       // returns 1
cache.put(3, 3);    // evicts key 2
cache.get(2);       // returns -1 (not found)
cache.put(4, 4);    // evicts key 1
cache.get(1);       // returns -1 (not found)
cache.get(3);       // returns 3
cache.get(4);       // returns 4

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思路： use a hashmap + doubly linked List: (head)..(pre)<—>(cur)<—>(next)..(end)
           Always set the head for the most recent accessed node.
           watch edge case like 1 node in list. Deleting head/tail, we need to set head/tail simultaneously.
Complexity: Time O(1) Space O(N)

public class LRUCache {
    class Node{
        Node pre;
        Node next;
        int val;
        int key;//need key fields for look back to delete in Map
        public Node(int key,int value){
            this.val = value;
            this.key = key;
        }
    }
    private void removeNode(Node n){
        Node preN = n.pre;
        Node nextN = n.next;
        if(n == head)head = nextN;//watch case of deleting head & end
        if(n == end)end = preN;//
        if(preN != null)preN.next = nextN;
        if(nextN != null)nextN.pre = preN;
    }
    public void setHead(Node n){//head is most recent val
        n.next = head;
        n.pre = null;
        if(head != null) head.pre = n;
        head = n;
        if(end == null)end = head;//actually when only head in list
    }
    //instance variable
    Node head = null;
    Node end = null;
    HashMap<Integer,Node> map = new HashMap<>(); //<val, node in list>
    int capacity;
    public LRUCache(int capacity) {
        this.capacity = capacity;
    }
    
    public int get(int key) {
        if(!map.containsKey(key))return -1;
        Node cur = map.get(key);
        removeNode(cur);
        setHead(cur);
        return cur.val;
    }
    
    public void put(int key, int value) {
        if(!map.containsKey(key)){
            if(map.size() >= capacity){
                map.remove(end.key);
                removeNode(end);
            }
            Node cur = new Node(key, value);
            map.put(key, cur);
            setHead(cur);
        }else{//update value
            Node cur = map.get(key);
            cur.val = value;
            removeNode(cur);
            setHead(cur);
        }
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * LRUCache obj = new LRUCache(capacity);
 * int param_1 = obj.get(key);
 * obj.put(key,value);
 */