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
键值对想到hashmap, 增删节点考虑链表, + 头尾节点, 因此要构造相应的类
public class LRUCache {
private class Node{
int key;
int value;
Node pre;
Node next;
public Node(int key, int value) {
this.key = key;
this.value = value;
this.pre = null;
this.next = null;
}
}
private int capacity;
private Node head = new Node(-1, -1);
private Node tail = new Node(-1, -1);
HashMap<Integer, Node> map = new HashMap<>();
public LRUCache(int capacity) {
this.capacity = capacity;
head.next = tail;
tail.pre = head;
}
public int get(int key) {
if (!map.containsKey(key)) {
return -1;
}
Node cur = map.get(key);
cur.pre.next = cur.next;
cur.next.pre = cur.pre;
moveToTail(cur);
return cur.value;
}
public void put(int key, int value) {
if (get(key) != -1) {
map.get(key).value = value;
return;
}
Node cur = new Node(key, value);
if (map.size() == capacity) {
map.remove(head.next.key);
head.next.next.pre = head;
head.next = head.next.next;
}
map.put(key, cur);
moveToTail(cur);
}
private void moveToTail(Node cur) {
tail.pre.next = cur;
cur.pre = tail.pre;
tail.pre = cur;
cur.next = tail;
}
}
/**
* 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);
*/
原文：http://www.cnblogs.com/apanda009/p/7242199.html