LeetCode-in-Rust

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 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.

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 <= 104
• 0 <= value <= 105
• At most 2 * 105 calls will be made to get and put.

Solution

struct LRUCache {
    map: HashMap<i32, Rc<RefCell<Node>>>,
    list: LinkedList,
    size: i32,
    capacity: i32,
}


/** 
 * `&self` means the method takes an immutable reference.
 * If you need a mutable reference, change it to `&mut self` instead.
 */
use std::cell::RefCell;
use std::collections::HashMap;
use std::rc::Rc;

struct Node {
    key: i32,
    value: i32,
    prev: Option<Rc<RefCell<Node>>>,
    next: Option<Rc<RefCell<Node>>>,
}

#[derive(Default)]
struct LinkedList {
    head: Option<Rc<RefCell<Node>>>,
    tail: Option<Rc<RefCell<Node>>>,
}

impl LinkedList {
    fn new() -> LinkedList {
        Default::default()
    }

    fn move_to_head(&mut self, node: &Rc<RefCell<Node>>) {
        let prev = node.borrow().prev.as_ref().map(|a| Rc::clone(a));
        let next = node.borrow().next.as_ref().map(|a| Rc::clone(a));
        match (&prev, &next) {
            (None, None) => {
                // this is the only node in list, so do nothng
            }
            (Some(_), None) => {
                // this node is already the head, so do nothing
            }
            (None, Some(next)) => {
                // this is the tail, move it to front
                node.borrow_mut().next = None;
                node.borrow_mut().prev = None;
                next.borrow_mut().prev = None;
                self.tail = Some(Rc::clone(next));

                let prev_head = self.head.as_ref().unwrap();
                prev_head.borrow_mut().next = Some(Rc::clone(node));
                node.borrow_mut().prev = Some(Rc::clone(prev_head));
                self.head = Some(Rc::clone(node));
            }
            (Some(prev), Some(next)) => {
                node.borrow_mut().next = None;
                node.borrow_mut().prev = None;

                prev.borrow_mut().next = Some(Rc::clone(next));
                next.borrow_mut().prev = Some(Rc::clone(prev));

                let prev_head = self.head.as_ref().unwrap();
                prev_head.borrow_mut().next = Some(Rc::clone(node));
                node.borrow_mut().prev = Some(Rc::clone(prev_head));
                self.head = Some(Rc::clone(node));
            }
        }
    }

    fn push_front(&mut self, node: &Rc<RefCell<Node>>) {
        match &self.head {
            None => {
                self.head = Some(Rc::clone(node));
                self.tail = Some(Rc::clone(node));
            }
            Some(prev_head) => {
                Rc::clone(node).borrow_mut().prev = Some(Rc::clone(prev_head));
                prev_head.borrow_mut().next = Some(Rc::clone(node));
                self.head = Some(Rc::clone(node));
            }
        }
    }

    fn remove_tail(&mut self) -> Option<Rc<RefCell<Node>>> {
        if let Some(tail) = self.tail.as_ref().map(|a| a.clone()) {
            if let Some(tail_next) = tail.borrow().next.as_ref() {
                tail_next.borrow_mut().prev = None;
                self.tail = Some(Rc::clone(tail_next));
            }
            tail.borrow_mut().next = None;
            tail.borrow_mut().prev = None;
            return Some(tail);
        }
        None
    }
}

impl LRUCache {
    fn new(capacity: i32) -> Self {
        Self {
            map: HashMap::new(),
            list: LinkedList::new(),
            size: 0,
            capacity,
        }
    }

    fn get(&mut self, key: i32) -> i32 {
        if let Some(node) = self.map.get(&key) {
            self.list.move_to_head(node);
            return node.borrow().value;
        } else {
            return -1;
        }
    }

    fn put(&mut self, key: i32, value: i32) {
        if let Some(node) = self.map.get(&key) {
            self.list.move_to_head(node);
            node.borrow_mut().value = value;
        } else {
            // remove tail node if needed
            if self.size >= self.capacity {
                if let Some(prev_tail) = self.list.remove_tail() {
                    self.map.remove(&prev_tail.borrow().key);
                };
            }
            // add node to list head
            let node = Rc::new(RefCell::new(Node {
                prev: None,
                next: None,
                key,
                value,
            }));
            self.list.push_front(&node);
            // update hashmap
            self.map.insert(key, node);
            // update size
            self.size += 1;
        }
    }
}

/**
 * Your LRUCache object will be instantiated and called as such:
 * let obj = LRUCache::new(capacity);
 * let ret_1: i32 = obj.get(key);
 * obj.put(key, value);
 */

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