Leetcode: LRU Cache

转载自点击打开链接

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.

使用map+双向链表，复杂度是O(logN)

链表头部的表示刚刚访问过的，链表尾部的表示很久之前访问的

每次get(key)，先在map中找到这个节点，然后把这个节点放到链表头部。

每次set(key, value)，现在map中找这个节点，如果有的话就把这个节点放到链表头部，如果没有就看看cache空间是否已经满了，size>=capacity，如果未满，就生成一个新的节点放到链表头部，如果满了，就生成一个新的节点放到链表头部并且删除链表尾部的一个节点。

struct node {
node* pre;
int key;
int value;
node* next;
node(int k, int v):key(k),value(v),pre(NULL),next(NULL) {};
};

class LRUCache {
map<int, node*> mp;
node* head;
node* tail;
int size;
int capacity;
public:
LRUCache(int c) {
if (c < 1)return;
head = new node(0, 0);
tail = new node(0, 0);
head->next = tail;
tail->pre = head;
mp.clear();
size = 0;
capacity = c;
}

int get(int k) {
map<int, node*>::iterator it = mp.find(k);
if (it != mp.end()) {
node* cur = (*it).second;
cur->pre->next = cur->next;
cur->next->pre = cur->pre;
putToHead(cur);
return cur->value;
} else
return -1;
}

void set(int k, int val) {
if (capacity < 1)return;
map<int, node*>::iterator it = mp.find(k);
if (it != mp.end()) {//find
node* cur = (*it).second;
cur->pre->next = cur->next;
cur->next->pre = cur->pre;
cur->value = val;
putToHead(cur);
} else {//not find
node* tmp = new node(k,val);
putToHead(tmp);
mp[k] = tmp;
if (size < capacity) {//size < capacity
size++;
} else {//size >= capacity
node* deltmp = tail->pre;
tail->pre = deltmp->pre;
deltmp->pre->next = tail;
it = mp.find(deltmp->key);
mp.erase(it);
delete deltmp;
}
}
}
void putToHead(node* cur)
{
cur->next = head->next;
cur->pre = head;
cur->next->pre = cur;
head->next = cur;
}

};

Hash+双向链表O(1)：

struct node{
node* pre;
int key;
int value;
node* next;
node(int k, int v):key(k),value(v),pre(NULL),next(NULL){};
};

class LRUCache{
unordered_map<int, node*> mp;
int capacity;
int size;
node* head;
node* tail;
public:
LRUCache(int c){
if(c<0)return;
head = new node(-1,-1);
tail = new node(-1,-1);
head->next = tail;
tail->pre = head;
mp.clear();
capacity = c;
size = 0;
}

int get(int k) {
unordered_map<int, node*>::iterator it = mp.find(k);
if(it != mp.end()){
node* p = it->second;
p->pre->next = p->next;
p->next->pre = p->pre;
putToHead(p);
return p->value;
}
else
return -1;
}

void set(int k, int val) {
if(capacity < 1) return;
unordered_map<int, node*>::iterator it = mp.find(k);
if(it != mp.end()){
node* p = it->second;
p->pre->next = p->next;
p->next->pre = p->pre;
putToHead(p);
p->value = val;
}else{
node* p = new node(k, val);
putToHead(p);
mp[k] = p;
size++;
if(size>capacity){
p = tail->pre;
tail->pre = p->pre;
p->pre->next = tail;
it = mp.find(p->key);
mp.erase(it);
delete p;
}
}
}

void putToHead(node* p)
{
p->next = head->next;
p->pre = head;
head->next->pre = p;
head->next = p;
}
};