我使用双向循环链表和 hashtable 完成了，这里有一个小细节就是 put 的时候也是访问，需要将对应的 Node 提取到 head 上。
Difficulty: Medium
Design and implement a data structure for . 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.
The cache is initialized with a positive capacity.
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
Language: Java
class LRUCache {
class LRUValue {
int key;
int value;
LRUValue next; //链表下一个节点的key
LRUValue pre;// 链表上一个节点的key
public LRUValue(int key, int value, LRUValue next, LRUValue pre) {
this.key = key;
this.value = value;
this.next = next;
this.pre = pre;
}
}
private Map<Integer, LRUValue> map = new HashMap<>();
private int capacity;
private LRUValue head;
public LRUCache(int capacity) {
this.capacity = capacity;
}
public int get(int key) {
if (!map.containsKey(key)) {
return -1;
} else {
LRUValue lruValue = map.get(key);
if (lruValue != head) { // 如果获取的元素不是在头部，需要将该元素升级到头部
promotionNode(lruValue);
}
return head.value;
}
}
public void put(int key, int value) {
if (map.size() == 0) {
LRUValue lruValue = new LRUValue(key, value, null, null);
lruValue.next = lruValue;
lruValue.pre = lruValue;
head = lruValue;
map.put(key, lruValue);
} else {
LRUValue targetValue = map.get(key);
if (targetValue != null) {// 注意 put 也算是访问，需要把它提到 header
if (targetValue != head) {
promotionNode(targetValue);
}
targetValue.value = value;
} else {
if (map.size() >= capacity) {
int removeKey = head.pre.key;
head.pre = head.pre.pre;
map.remove(removeKey);
}
LRUValue lruValue = new LRUValue(key, value, head, head.pre);
map.put(key, lruValue);
head.pre.next = lruValue;
head.pre = lruValue;
head = lruValue;
}
}
}
private void promotionNode(LRUValue lruValue) {
lruValue.pre.next = lruValue.next;
lruValue.next.pre = lruValue.pre;
head.pre.next = lruValue;
lruValue.next = head;
lruValue.pre = head.pre;
head.pre = lruValue;
head = lruValue;
}
}
/**
* 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);
*/