[Medium] 146. LRU Cache

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 size capacity.
int get(int key) Return the value of the key if the key exists, otherwise return -1.
void put(int key, int value) Update the value of the key if the key exists. Otherwise, add the key-value pair to the cache. If the number of keys exceeds the capacity from this operation, evict the least recently used key.

The functions get and put must each run in O(1) average time complexity.

Examples

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 <= 3000
0 <= key <= 10^4
0 <= value <= 10^5
At most 2 * 10^5 calls will be made to get and put.

Clarification Questions

Before diving into the solution, here are 5 important clarifications and assumptions to discuss during an interview:

LRU definition: What does “Least Recently Used” mean? (Assumption: The item that hasn’t been accessed for the longest time - need to track access order)
Cache operations: What operations should the cache support? (Assumption: get(key) - retrieve value, put(key, value) - insert/update, both operations mark item as recently used)
Eviction policy: When should we evict items? (Assumption: When cache is full and we need to add a new item, evict the least recently used item)
Capacity: What is the cache capacity? (Assumption: Fixed capacity specified in constructor - cannot exceed this limit)
Return values: What should get() return if key doesn’t exist? (Assumption: Return -1 - key not found in cache)

Interview Deduction Process (20 minutes)

Step 1: Brute-Force Approach (5 minutes)

Use a hash map to store key-value pairs and a list/array to track access order. For get, search the list to find the key, move it to the end, and return the value. For put, add/update the hash map and move the key to the end of the list. When capacity is exceeded, remove the first element from the list. This approach has O(n) time for get operations due to list searching, which doesn’t meet the O(1) requirement.

Step 2: Semi-Optimized Approach (7 minutes)

Use a hash map and a doubly linked list: hash map stores key-to-node mappings, doubly linked list maintains access order. For get, use hash map to find the node in O(1), move it to the end of the list. For put, add/update hash map, add node to end of list, remove head if capacity exceeded. However, implementing a doubly linked list from scratch requires careful pointer management and can be error-prone.

Step 3: Optimized Solution (8 minutes)

Use hash map + std::list: hash map stores key -> iterator mappings, std::list maintains access order with pairs (key, value). For get, use hash map to get iterator in O(1), use splice to move node to end in O(1). For put, if key exists, update value and move to end. If new key, add to end and remove head if capacity exceeded. This achieves O(1) for both operations using standard library containers. The key insight is that std::list::splice allows O(1) node movement, and hash map provides O(1) lookup, combining to achieve O(1) operations.

Solution: Hash Map + Doubly Linked List (C++20 Optimized)

Time Complexity: O(1) for both get and put
Space Complexity: O(capacity)

We use a combination of hash map and doubly linked list to achieve O(1) operations. The hash map stores key-to-node mappings, and the doubly linked list maintains the order of recently used items.

Solution 1: Using list (Recommended - C++20 Optimized)

using namespace std;

class LRUCache {
private:
    int capacity_;
    unordered_map<int, list<pair<int, int>>::iterator> cache_;
    list<pair<int, int>> lru_list_;

    // Helper to move node to front (most recently used)
    void moveToFront(list<pair<int, int>>::iterator it) {
        if (it != lru_list_.begin()) {
            lru_list_.splice(lru_list_.begin(), lru_list_, it);
        }
    }

public:
    explicit LRUCache(int capacity) 
        : capacity_(capacity) 
    {
        cache_.reserve(capacity_);  // Pre-allocate hash map
    }

    int get(int key) {
        auto it = cache_.find(key);
        if (it == cache_.end()) {
            return -1;
        }

        // Move to front (most recently used)
        moveToFront(it->second);
        return it->second->second;
    }

    void put(int key, int value) {
        auto it = cache_.find(key);
        
        if (it != cache_.end()) {
            // Update existing key
            it->second->second = value;
            moveToFront(it->second);
        } else {
            // Add new key
            if (cache_.size() >= capacity_) {
                // Evict least recently used (back of list)
                auto [lru_key, _] = lru_list_.back();
                cache_.erase(lru_key);
                lru_list_.pop_back();
            }
            
            // Insert at front
            lru_list_.emplace_front(key, value);
            cache_[key] = lru_list_.begin();
        }
    }
};

Solution 2: Custom Doubly Linked List (C++20 Optimized)

using namespace std;

class LRUCache {
private:
    struct Node {
        int key;
        int value;
        Node* next;
        Node* prev;
        
        Node(int k, int v) 
            : key(k), value(v), next(nullptr), prev(nullptr) {}
    };

    int capacity_;
    unordered_map<int, Node*> cache_;
    
    // Dummy head and tail for easier list manipulation
    unique_ptr<Node> head_;
    unique_ptr<Node> tail_;

    // Add node right before tail (most recently used)
    void addNode(Node* node) {
        Node* prev_end = tail_->prev;
        prev_end->next = node;
        node->prev = prev_end;
        node->next = tail_.get();
        tail_->prev = node;
    }

    // Remove node from list
    void removeNode(Node* node) {
        node->prev->next = node->next;
        node->next->prev = node->prev;
    }

    // Move node to end (most recently used)
    void moveToEnd(Node* node) {
        removeNode(node);
        addNode(node);
    }

public:
    explicit LRUCache(int capacity) 
        : capacity_(capacity)
        , head_(make_unique<Node>(-1, -1))
        , tail_(make_unique<Node>(-1, -1))
    {
        head_->next = tail_.get();
        tail_->prev = head_.get();
        cache_.reserve(capacity_);
    }

    ~LRUCache() {
        // Clean up nodes
        Node* current = head_->next;
        while (current != tail_.get()) {
            Node* next = current->next;
            delete current;
            current = next;
        }
    }

    // Delete copy constructor and assignment
    LRUCache(const LRUCache&) = delete;
    LRUCache& operator=(const LRUCache&) = delete;

    int get(int key) {
        auto it = cache_.find(key);
        if (it == cache_.end()) {
            return -1;
        }

        Node* node = it->second;
        moveToEnd(node);
        return node->value;
    }

    void put(int key, int value) {
        auto it = cache_.find(key);
        
        if (it != cache_.end()) {
            // Update existing
            Node* node = it->second;
            node->value = value;
            moveToEnd(node);
        } else {
            // Add new
            if (cache_.size() >= capacity_) {
                // Evict least recently used (head->next)
                Node* lru = head_->next;
                removeNode(lru);
                cache_.erase(lru->key);
                delete lru;
            }
            
            Node* newNode = new Node(key, value);
            addNode(newNode);
            cache_[key] = newNode;
        }
    }
};

Solution 3: Most Optimized with Move Semantics

#include <unordered_map>
#include <list>
#include <utility>

class LRUCache {
private:
    int capacity_;
    std::unordered_map<int, std::list<std::pair<int, int>>::iterator> cache_;
    std::list<std::pair<int, int>> lru_list_;

public:
    explicit LRUCache(int capacity) 
        : capacity_(capacity) 
    {
        cache_.reserve(capacity_);
    }

    [[nodiscard]] int get(int key) {
        const auto it = cache_.find(key);
        if (it == cache_.end()) {
            return -1;
        }

        // Move to front using splice (O(1))
        lru_list_.splice(lru_list_.begin(), lru_list_, it->second);
        return it->second->second;
    }

    void put(int key, int value) {
        auto it = cache_.find(key);
        
        if (it != cache_.end()) {
            // Update and move to front
            it->second->second = value;
            lru_list_.splice(lru_list_.begin(), lru_list_, it->second);
        } else {
            // Check capacity
            if (cache_.size() >= capacity_) {
                // Evict LRU (back of list)
                cache_.erase(lru_list_.back().first);
                lru_list_.pop_back();
            }
            
            // Insert at front
            lru_list_.emplace_front(key, value);
            cache_[key] = lru_list_.begin();
        }
    }
};

Key Optimizations (C++20)

list::splice(): O(1) operation to move nodes without copying
unordered_map::reserve(): Pre-allocates hash map to avoid rehashing
explicit constructor: Prevents implicit conversions
Structured bindings: Cleaner code with auto [key, value]
emplace_front(): Constructs in-place, avoiding copies
Move semantics: Efficient transfer of ownership

How the Algorithm Works

Data Structure Design

Hash Map:          Doubly Linked List:
key -> iterator    [head] <-> [1,1] <-> [2,2] <-> [tail]
                   (LRU)                (MRU)

Operation Flow

Get Operation:

Look up key in hash map → O(1)
If found, move node to front (most recently used) → O(1)
Return value

Put Operation:

Look up key in hash map → O(1)
If exists: update value and move to front → O(1)
If new:
- Check capacity
- If full: remove back node (LRU) → O(1)
- Insert at front → O(1)

Example Walkthrough

capacity = 2

put(1, 1):  cache = {1: [1,1]}
            list: [head] <-> [1,1] <-> [tail]

put(2, 2):  cache = {1: [1,1], 2: [2,2]}
            list: [head] <-> [1,1] <-> [2,2] <-> [tail]

get(1):     Move [1,1] to front
            list: [head] <-> [2,2] <-> [1,1] <-> [tail]
            return 1

put(3, 3):  Evict [2,2] (LRU), add [3,3] at front
            cache = {1: [1,1], 3: [3,3]}
            list: [head] <-> [3,3] <-> [1,1] <-> [tail]

Complexity Analysis

Operation	Time	Space
`get(key)`	O(1)	O(1)
`put(key, value)`	O(1)	O(1)
Overall	O(1)	O(capacity)

Why std::list is Preferred

splice() is O(1): Moves nodes without copying
Automatic memory management: No manual node deletion
Less error-prone: No pointer management
Better cache locality: Standard library optimizations
Cleaner code: Less boilerplate

Edge Cases

Capacity = 1: Only one item can exist
Get non-existent key: Returns -1
Update existing key: Moves to front, doesn’t increase size
Multiple puts: Evicts oldest when capacity exceeded

Common Mistakes

Not moving to front on get: Must update access order
Wrong eviction order: Remove from back (LRU), not front
Memory leaks: Forgetting to delete nodes in custom implementation
Not updating iterator: After list modification, iterators may be invalid
Copying instead of moving: Use splice() or move semantics

Robina Li

[Medium] 146. LRU Cache

[Medium] 146. LRU Cache

Examples

Constraints

Clarification Questions

Interview Deduction Process (20 minutes)

Solution: Hash Map + Doubly Linked List (C++20 Optimized)

Solution 1: Using list (Recommended - C++20 Optimized)

Solution 2: Custom Doubly Linked List (C++20 Optimized)

Solution 3: Most Optimized with Move Semantics

Key Optimizations (C++20)

How the Algorithm Works

Data Structure Design

Operation Flow

Example Walkthrough

Complexity Analysis

Why std::list is Preferred

Edge Cases

Common Mistakes

Related Posts

Recent Posts

[Medium] 146. LRU Cache

Examples

Constraints

Clarification Questions

Interview Deduction Process (20 minutes)

Solution: Hash Map + Doubly Linked List (C++20 Optimized)

Solution 1: Using list (Recommended - C++20 Optimized)

Solution 2: Custom Doubly Linked List (C++20 Optimized)

Solution 3: Most Optimized with Move Semantics

Key Optimizations (C++20)

How the Algorithm Works

Data Structure Design

Operation Flow

Example Walkthrough

Complexity Analysis

Why std::list is Preferred

Edge Cases

Common Mistakes

Related Problems

Related Posts

Recent Posts