LeetCode-in-Java
146. LRU Cache
Medium
Design a data structure that follows the constraints of a Least Recently Used (LRU) cache.
Implement the LRUCache class:
LRUCache(int capacity)Initialize the LRU cache with positive sizecapacity.int get(int key)Return the value of thekeyif the key exists, otherwise return-1.void put(int key, int value)Update the value of thekeyif thekeyexists. Otherwise, add thekey-valuepair to the cache. If the number of keys exceeds thecapacityfrom this operation, evict the least recently used key.
The functions get and put must each run in O(1) average time complexity.
Example 1:
Input [“LRUCache”, “put”, “put”, “get”, “put”, “get”, “put”, “get”, “get”, “get”] [[2], [1, 1], [2, 2], [1], [3, 3], [2], [4, 4], [1], [3], [4]]
Output: [null, null, null, 1, null, -1, null, -1, 3, 4]
Explanation:
LRUCache lRUCache = new LRUCache(2);
lRUCache.put(1, 1); // cache is {1=1}
lRUCache.put(2, 2); // cache is {1=1, 2=2}
lRUCache.get(1); // return 1
lRUCache.put(3, 3); // LRU key was 2, evicts key 2, cache is {1=1, 3=3}
lRUCache.get(2); // returns -1 (not found)
lRUCache.put(4, 4); // LRU key was 1, evicts key 1, cache is {4=4, 3=3}
lRUCache.get(1); // return -1 (not found)
lRUCache.get(3); // return 3
lRUCache.get(4); // return 4
Constraints:
1 <= capacity <= 30000 <= key <= 1040 <= value <= 105- At most 2
* 105calls will be made togetandput.
Solution
import java.util.HashMap;
import java.util.Map;
public class LRUCache {
private static class LruCacheNode {
int key;
int value;
LruCacheNode prev;
LruCacheNode next;
public LruCacheNode(int k, int v) {
key = k;
value = v;
}
}
private int capacity;
private Map<Integer, LruCacheNode> cacheMap = new HashMap<>();
// insert here
private LruCacheNode head;
// remove here
private LruCacheNode tail;
public LRUCache(int cap) {
capacity = cap;
}
public int get(int key) {
LruCacheNode val = cacheMap.get(key);
if (val == null) {
return -1;
}
moveToHead(val);
return val.value;
}
/*
* Scenarios :
* 1. Value key is already present.
* update
* move node to head
* 2. cache is not full
* cache is empty (create node and assign head and tail)
* cache is partially empty (add node to head and update head pointer)
* 3. cache is full
* remove node at tail, update head, tail pointers
* Recursively call put
*
*
* move node to head Scenarios
* 1. node is at head
* 2. node is at tail
* 3. node is in middle
*
*/
public void put(int key, int value) {
LruCacheNode valNode = cacheMap.get(key);
if (valNode != null) {
valNode.value = value;
moveToHead(valNode);
} else {
if (cacheMap.size() < capacity) {
if (cacheMap.size() == 0) {
LruCacheNode node = new LruCacheNode(key, value);
cacheMap.put(key, node);
head = node;
tail = node;
} else {
LruCacheNode node = new LruCacheNode(key, value);
cacheMap.put(key, node);
node.next = head;
head.prev = node;
head = node;
}
} else {
// remove from tail
LruCacheNode last = tail;
tail = last.prev;
if (tail != null) {
tail.next = null;
}
cacheMap.remove(last.key);
if (cacheMap.size() == 0) {
head = null;
}
// Call recursively
put(key, value);
}
}
}
/*
* check for 3 conditions
* 1. node is already at head
* 2. Node is tail node
* 3. Node in middle node
*/
private void moveToHead(LruCacheNode node) {
if (node == head) {
return;
}
if (node == tail) {
tail = node.prev;
}
// node is not head, it should have some valid prev node
LruCacheNode prev = node.prev;
LruCacheNode next = node.next;
prev.next = next;
if (next != null) {
next.prev = prev;
}
node.prev = null;
node.next = head;
head.prev = node;
head = node;
}
}
/*
* 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);
*/
Time Complexity (Big O Time):
get(int key)Method:- Accessing an element from the
cacheMapis done in constant time (O(1)). - Moving a node to the head in the
moveToHeadmethod also operates in constant time (O(1)). - Overall, the
getmethod’s time complexity is O(1).
- Accessing an element from the
put(int key, int value)Method:- Updating the value of an existing key in the
cacheMapis done in constant time (O(1)). - Adding a new node to the cache is done in constant time if the cache is not full (O(1)).
- If the cache is full, it removes the node at the tail, which is also done in constant time (O(1)).
- Recursively calling the
putmethod when the cache is full does not result in nested loops because the eviction operation happens once in each recursion. - Overall, the
putmethod’s time complexity is O(1) on average, but in the worst case (when eviction happens multiple times), it can be considered O(N), where N is the capacity of the cache.
- Updating the value of an existing key in the
moveToHead(LruCacheNode node)Method:- Moving a node to the head of the cache is done in constant time (O(1)) because it involves changing pointers.
- The time complexity of this method is O(1).
Overall, the time complexity of most operations in the LRU cache is O(1), except for the put method in the worst-case scenario.
Space Complexity (Big O Space):
The space complexity of this LRU cache implementation is O(capacity), where “capacity” represents the maximum number of key-value pairs the cache can store. The primary space usage comes from the cacheMap, which stores the key-value pairs, and the linked list of nodes (head and tail) used to maintain the order of access. Additionally, each node in the linked list consumes space proportional to the key and value sizes.
In summary, the space complexity of the LRU cache is determined by its capacity, and the primary space-consuming data structure is the cacheMap, resulting in an overall space complexity of O(capacity).