Design and implement a data structure for Least Recently Used (LRU) cache. It should support the following operations: get and set.
get(key) - Get the value (will always be positive) of the key if the key exists in the cache, otherwise return -1.set(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.
设计一个近期最少使用页面置换缓存LRU(Least Recently Used)，实现get(key), set(key, value)功能。
get(key)：取值(key恒为正), 不存在时返回-1。如果存在，返回值，并且delete此key，在从新写入cache，因为要最近刚使用过，要把它放到队尾。
set(key, value)：缓存已满，删除近期最久未被使用的节点，添加新节点进缓存。缓存未满，节点存在，修改value；节点不存在，添加新节点进缓存；最后更新此节点到队尾。
解法1: 双向链表(Doubly-Linked List) + HashMap
双向链表：维护缓存节点CacheNode，凡是被访问（新建/修改命中/访问命中）过的节点，一律在访问完成后移动到双向链表尾部，保证链表尾部始终为最新节点；保证链表头部始终为最旧节点，LRU策略删除时表现为删除双向链表头部；由于链表不支持随机访问，使用HashMap+双向链表实现LRU缓存，HashMap中键值对：<key, CacheNode>。
解法2: OrderedDict有序字典
Time: Get O(1) Set O(1), Space: O(N)
Java：
import java.util.HashMap;
class Solution {
private HashMap<Integer, CacheNode> map;
private int capacity;
// head.next和tail.next指向链表头尾，包起来防止null
private CacheNode head = new CacheNode(-1, -1);
private CacheNode tail = new CacheNode(-1, -1);
private class CacheNode {
int key, value;
CacheNode pre, next;
CacheNode(int key, int value) {
this.key = key;
this.value = value;
this.pre = null;
this.next = null;
}
}
public Solution(int capacity) {
this.map = new HashMap<>();
this.capacity = capacity;
}
// 将已有节点或新建节点移动到链表尾部
private void moveToTail(CacheNode target, boolean isNew) {
// 尾部节点显然不需要移动
if (target != tail.next) {
if (!isNew) {
// 修改旧节点的双向链表指针
target.pre.next = target.next;
target.next.pre = target.pre;
}
// 添加节点到链表尾部
tail.next.next = target;
target.pre = tail.next;
tail.next = target;
}
}
// 命中节点添加到链表尾部，未命中返回-1
public int get(int key) {
if (map.containsKey(key)) {
CacheNode target = map.get(key);
// 将已有节点移动到链表尾部
moveToTail(target, false);
// 此时链表尾部tail.next = target，更新next指向null，防止出现环
tail.next.next = null;
return target.value;
}
return -1;
}
public void set(int key, int value) {
if (map.containsKey(key)) {
CacheNode target = map.get(key);
target.value = value;
map.put(key, target);
// 将访问过的已有节点移动到链表尾部
moveToTail(target, false);
} else if(map.size() < capacity) { // cache未满，添加节点
CacheNode newNode = new CacheNode(key, value);
map.put(key, newNode);
if (head.next == null) {
head.next = newNode;
newNode.pre = head;
tail.next = newNode;
} else {
// 将新建节点移动到链表尾部
moveToTail(newNode, true);
}
} else { // cache已满，淘汰链表链表头部节点，新节点加入到链表尾部
CacheNode newNode = new CacheNode(key, value);
map.remove(head.next.key);
map.put(key, newNode);
// cache中只有一个元素
if (head.next == tail.next) {
head.next = newNode;
tail.next = newNode;
} else { // cache中不止一个元素，删除头部
head.next.next.pre = head; // 更新新头部.pre = head
head.next = head.next.next;// 更新新头部
// 将新建节点移动到链表尾部
moveToTail(newNode, true);
}
}
}
}
Python:
class Node:
def __init__(self, key, val):
self.key = key
self.val = val
self.prev = None
self.next = None
class LRUCache:
# @param capacity, an integer
def __init__(self, capacity):
self.capacity = capacity
self.size = 0
self.dummyNode = Node(-1, -1)
self.tail = self.dummyNode
self.entryFinder = {}
# @return an integer
def get(self, key):
entry = self.entryFinder.get(key)
if entry is None:
return -1
else:
self.renew(entry)
return entry.val
# @param key, an integer
# @param value, an integer
# @return nothing
def set(self, key, value):
entry = self.entryFinder.get(key)
if entry is None:
entry = Node(key, value)
self.entryFinder[key] = entry
self.tail.next = entry
entry.prev = self.tail
self.tail = entry
if self.size < self.capacity:
self.size += 1
else:
headNode = self.dummyNode.next
if headNode is not None:
self.dummyNode.next = headNode.next
headNode.next.prev = self.dummyNode
del self.entryFinder[headNode.key]
else:
entry.val = value
self.renew(entry)
def renew(self, entry):
if self.tail != entry:
prevNode = entry.prev
nextNode = entry.next
prevNode.next = nextNode
nextNode.prev = prevNode
entry.next = None
self.tail.next = entry
entry.prev = self.tail
self.tail = entry
Python:
class ListNode(object):
def __init__(self, key, val):
self.val = val
self.key = key
self.next = None
self.prev = None
class LinkedList(object):
def __init__(self):
self.head = None
self.tail = None
def insert(self, node):
node.next, node.prev = None, None # avoid dirty node
if self.head is None:
self.head = node
else:
self.tail.next = node
node.prev = self.tail
self.tail = node
def delete(self, node):
if node.prev:
node.prev.next = node.next
else:
self.head = node.next
if node.next:
node.next.prev = node.prev
else:
self.tail = node.prev
node.next, node.prev = None, None # make node clean
class LRUCache(object):
def __init__(self, capacity):
self.list = LinkedList()
self.dict = {}
self.capacity = capacity
def _insert(self, key, val):
node = ListNode(key, val)
self.list.insert(node)
self.dict[key] = node
def get(self, key):
if key in self.dict:
val = self.dict[key].val
self.list.delete(self.dict[key])
self._insert(key, val)
return val
return -1
def set(self, key, val):
if key in self.dict:
self.list.delete(self.dict[key])
elif len(self.dict) == self.capacity:
del self.dict[self.list.head.key]
self.list.delete(self.list.head)
self._insert(key, val)
Python:
class LRUCache:
def __init__(self, capacity):
self.capacity = capacity
self.cache = collections.OrderedDict()
def get(self, key):
if not key in self.cache:
return -1
value = self.cache.pop(key)
self.cache[key] = value
return value
def set(self, key, value):
if key in self.cache:
self.cache.pop(key)
elif len(self.cache) == self.capacity:
self.cache.popitem(last=False)
self.cache[key] = value
C++:
class LRUCache{
public:
LRUCache(int capacity) {
cap = capacity;
}
int get(int key) {
auto it = m.find(key);
if (it == m.end()) return -1;
l.splice(l.begin(), l, it->second);
return it->second->second;
}
void set(int key, int value) {
auto it = m.find(key);
if (it != m.end()) l.erase(it->second);
l.push_front(make_pair(key, value));
m[key] = l.begin();
if (m.size() > cap) {
int k = l.rbegin()->first;
l.pop_back();
m.erase(k);
}
}
private:
int cap;
list<pair<int, int> > l;
unordered_map<int, list<pair<int, int> >::iterator> m;
};
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