2622. Cache With Time Limit

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2622. Cache With Time Limit

題目描述

Write a class that allows getting and setting key-value pairs, however a time until expiration is associated with each key.

The class has three public methods:

set(key, value, duration): accepts an integer key, an integer value, and a duration in milliseconds. Once the duration has elapsed, the key should be inaccessible. The method should return true if the same un-expired key already exists and false otherwise. Both the value and duration should be overwritten if the key already exists.

get(key): if an un-expired key exists, it should return the associated value. Otherwise it should return -1.

count(): returns the count of un-expired keys.

範例

Example 1:

Input: 
["TimeLimitedCache", "set", "get", "count", "get"]
[[], [1, 42, 100], [1], [], [1]]
[0, 0, 50, 50, 150]
Output: [null, false, 42, 1, -1]
Explanation:
At t=0, the cache is constructed.
At t=0, a key-value pair (1: 42) is added with a time limit of 100ms. The value doesn't exist so false is returned.
At t=50, key=1 is requested and the value of 42 is returned.
At t=50, count() is called and there is one active key in the cache.
At t=100, key=1 expires.
At t=150, get(1) is called but -1 is returned because the cache is empty.

Example 2:

Input: 
["TimeLimitedCache", "set", "set", "get", "get", "get", "count"]
[[], [1, 42, 50], [1, 50, 100], [1], [1], [1], []]
[0, 0, 40, 50, 120, 200, 250]
Output: [null, false, true, 50, 50, -1]
Explanation:
At t=0, the cache is constructed.
At t=0, a key-value pair (1: 42) is added with a time limit of 50ms. The value doesn't exist so false is returned.
At t=40, a key-value pair (1: 50) is added with a time limit of 100ms. A non-expired value already existed so true is returned and the old value was overwritten.
At t=50, get(1) is called which returned 50.
At t=120, get(1) is called which returned 50.
At t=140, key=1 expires.
At t=200, get(1) is called but the cache is empty so -1 is returned.
At t=250, count() returns 0 because the cache is empty.

Constraints:

• 0 <= key <= 10⁹
• 0 <= value <= 10⁹
• 0 <= duration <= 1000
• total method calls will not exceed 100

解答

TypeScript

class TimeLimitedCache {
  private cache: {
    [key: number]: {
      value: number;
      expiredTime: number;
    };
  };
  constructor() {
    this.cache = {};
  }

  set(key: number, value: number, duration: number): boolean {
    const currentTime = Date.now();
    const existed = this.cache[key] ? this.cache[key].expiredTime > currentTime : false;

    this.cache[key] = {
      value,
      expiredTime: Date.now() + duration,
    };
    return existed;
  }

  get(key: number): number {
    const currentTime = Date.now();
    if (this.cache[key]?.expiredTime > currentTime) {
      return this.cache[key].value;
    }
    return -1;
  }

  count(): number {
    const currentTime = Date.now();
    let counts = 0;
    for (const value of Object.values(this.cache)) {
      if (value.expiredTime - currentTime > 0) {
        counts++;
      }
    }
    return counts;
  }
}

setTimeout + clearTimeout 寫法：

class TimeLimitedCache {
  cache = new Map<number, { value: number; timeout: NodeJS.Timeout }>();

  set(key: number, value: number, duration: number) {
    const valueInCache = this.cache.get(key);
    if (valueInCache) {
      clearTimeout(valueInCache.timeout);
    }
    const timeout = setTimeout(() => this.cache.delete(key), duration);
    this.cache.set(key, { value, timeout });
    return Boolean(valueInCache);
  }

  get(key: number) {
    return this.cache.has(key) ? this.cache.get(key)?.value : -1;
  }

  count() {
    return this.cache.size;
  }
}

Priority Queue

import { MinPriorityQueue } from '@datastructures-js/priority-queue';

type Entry = {
  key: number;
  value: number;
  expiration: number;
  overwritten: boolean;
};

class TimeLimitedCache {
  cache: Record<string, Entry> = {};
  queue = new MinPriorityQueue<Entry>();
  size = 0;

  handleExpiredData() {
    const now = Date.now();
    while (this.queue.size() > 0 && this.queue.front().expiration < now) {
      const entry = this.queue.dequeue();
      if (!entry.overwritten) {
        delete this.cache[entry.key];
        this.size -= 1;
      }
    }
  }

  set(key: number, value: number, duration: number) {
    this.handleExpiredData();
    const hasVal = key in this.cache;
    if (hasVal) {
      this.cache[key].overwritten = true;
    } else {
      this.size += 1;
    }
    const expiration = Date.now() + duration;
    const entry: Entry = { key, value, expiration, overwritten: false };
    this.cache[key] = entry;
    this.queue.enqueue(entry);
    return hasVal;
  }

  get(key: number) {
    this.handleExpiredData();
    if (this.cache[key] === undefined) return -1;
    return this.cache[key].value;
  }

  count() {
    this.handleExpiredData();
    return this.size;
  }
}

SheepThur, May 18, 2023

Reference

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