Lfu cache

Hash Table Linked List Design Doubly-Linked List

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Jan 13, 2026

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Design and implement a data structure for a Least Frequently Used (LFU) cache. Implement the LFUCache class: LFUCache(int capacity) Initializes the object with the capacity of the data structure. int get(int key) Gets the value of the key if the key exists in the cache. Otherwise, returns -1. void put(int key, int value) Update the value of the key if present, or inserts the key if not already present. When the cache reaches its capacity, it should invalidate and remove the least frequently used key before inserting a new item. For this problem, when there is a tie (i.e., two or more keys with the same frequency), the least recently used key would be invalidated. To determine the least frequently used key, a use counter is maintained for each key in the cache. The key with the smallest use counter is the least frequently used key. When a key is first inserted into the cache, its use counter is set to 1 (due to the put operation). The use counter for a key in the cache is incremented either a get or put operation is called on it. The functions get and put must each run in O(1) average time complexity.

Solution

Rust

Time O(n)

Space O(n)

LeetCode

solution.rs

use std::cell::RefCell;
use std::collections::{HashMap, VecDeque};

struct Node {
  #[allow(dead_code)]
  key: i32,
  value: i32,
  freq: usize,
}

pub struct LFUCache {
  capacity: usize,
  min_freq: usize,
  key_to_node: RefCell<HashMap<i32, Node>>,
  freq_to_keys: RefCell<HashMap<usize, VecDeque<i32>>>,
}

impl LFUCache {
  pub fn new(capacity: i32) -> Self {
    LFUCache {
      capacity: capacity as usize,
      min_freq: 0,
      key_to_node: RefCell::new(HashMap::new()),
      freq_to_keys: RefCell::new(HashMap::new()),
    }
  }

  pub fn get(&self, key: i32) -> i32 {
    let mut key_to_node = self.key_to_node.borrow_mut();

    if let Some(node) = key_to_node.get(&key) {
      let value = node.value;
      let freq = node.freq;

      // Remove from current frequency list
      let mut freq_to_keys = self.freq_to_keys.borrow_mut();
      if let Some(keys) = freq_to_keys.get_mut(&freq) {
        if let Some(pos) = keys.iter().position(|&k| k == key) {
          keys.remove(pos);
        }
        if keys.is_empty() {
          freq_to_keys.remove(&freq);
          if freq == self.min_freq {
            let new_min = freq + 1;
            let self_ptr = self as *const Self as *mut Self;
            unsafe {
              (*self_ptr).min_freq = new_min;
            }
          }
        }
      }

      // Update node frequency
      let node = key_to_node.get_mut(&key).unwrap();
      node.freq += 1;
      let new_freq = node.freq;

      // Add to new frequency list
      freq_to_keys
        .entry(new_freq)
        .or_insert_with(VecDeque::new)
        .push_back(key);

      value
    } else {
      -1
    }
  }

  pub fn put(&self, key: i32, value: i32) {
    if self.capacity == 0 {
      return;
    }

    let mut key_to_node = self.key_to_node.borrow_mut();

    if let Some(node) = key_to_node.get(&key) {
      let freq = node.freq;

      // Remove from current frequency list
      let mut freq_to_keys = self.freq_to_keys.borrow_mut();
      if let Some(keys) = freq_to_keys.get_mut(&freq) {
        if let Some(pos) = keys.iter().position(|&k| k == key) {
          keys.remove(pos);
        }
        if keys.is_empty() {
          freq_to_keys.remove(&freq);
          if freq == self.min_freq {
            let new_min = freq + 1;
            let self_ptr = self as *const Self as *mut Self;
            unsafe {
              (*self_ptr).min_freq = new_min;
            }
          }
        }
      }

      // Update node
      let node = key_to_node.get_mut(&key).unwrap();
      node.value = value;
      node.freq += 1;
      let new_freq = node.freq;

      // Add to new frequency list
      freq_to_keys
        .entry(new_freq)
        .or_insert_with(VecDeque::new)
        .push_back(key);
    } else {
      // Evict if at capacity
      if key_to_node.len() >= self.capacity {
        let mut freq_to_keys = self.freq_to_keys.borrow_mut();
        if let Some(keys) = freq_to_keys.get_mut(&self.min_freq) {
          if let Some(evict_key) = keys.pop_front() {
            key_to_node.remove(&evict_key);
            if keys.is_empty() {
              freq_to_keys.remove(&self.min_freq);
            }
          }
        }
      }

      // Insert new node
      key_to_node.insert(
        key,
        Node {
          key,
          value,
          freq: 1,
        },
      );
      let mut freq_to_keys = self.freq_to_keys.borrow_mut();
      freq_to_keys
        .entry(1)
        .or_insert_with(VecDeque::new)
        .push_back(key);

      let self_ptr = self as *const Self as *mut Self;
      unsafe {
        (*self_ptr).min_freq = 1;
      }
    }
  }
}