webDevInterviewQuestions/algorithm/04-practical-scenarios.md

# 实际场景与逻辑题

## 1. 并发控制（限制并发请求数）

```js
async function limitConcurrency(tasks, limit) {
  const results = [];
  const executing = [];
  
  for (const [index, task] of tasks.entries()) {
    const p = Promise.resolve().then(() => task()).then(res => {
      results[index] = res;
      executing.splice(executing.indexOf(p), 1);
    });
    executing.push(p);
    
    if (executing.length >= limit) {
      await Promise.race(executing);
    }
  }
  
  await Promise.all(executing);
  return results;
}

// 使用示例
const tasks = urls.map(url => () => fetch(url));
await limitConcurrency(tasks, 3); // 最多3个并发
```

### 异步任务调度器

```js
class Scheduler {
  constructor(limit) {
    this.limit = limit;
    this.queue = [];
    this.running = 0;
  }
  
  add(promiseCreator) {
    return new Promise((resolve, reject) => {
      this.queue.push({ promiseCreator, resolve, reject });
      this.run();
    });
  }
  
  run() {
    while (this.running < this.limit && this.queue.length) {
      const { promiseCreator, resolve, reject } = this.queue.shift();
      this.running++;
      promiseCreator()
        .then(resolve, reject)
        .finally(() => { this.running--; this.run(); });
    }
  }
}
```

---

## 2. 缓存函数（Memoization）

```js
function memoize(fn) {
  const cache = new Map();
  return function(...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

// 支持过期时间
function memoizeWithTTL(fn, ttl = 60000) {
  const cache = new Map();
  return function(...args) {
    const key = JSON.stringify(args);
    const cached = cache.get(key);
    if (cached && Date.now() - cached.time < ttl) {
      return cached.value;
    }
    const result = fn.apply(this, args);
    cache.set(key, { value: result, time: Date.now() });
    return result;
  };
}
```

---

## 3. 解析 URL 参数

```js
function parseQuery(url) {
  const query = url.split('?')[1] || '';
  return query.split('&').reduce((acc, pair) => {
    const [key, value] = pair.split('=').map(decodeURIComponent);
    if (key) {
      acc[key] = acc[key] 
        ? [].concat(acc[key], value) 
        : value;
    }
    return acc;
  }, {});
}

// 使用正则
function parseQueryRegex(url) {
  const result = {};
  url.replace(/[?&]([^=&#]+)=([^&#]*)/g, (_, key, value) => {
    result[decodeURIComponent(key)] = decodeURIComponent(value);
  });
  return result;
}

// 示例: "https://example.com?a=1&b=2&a=3"
// => { a: ['1', '3'], b: '2' }
```

---

## 4. DOM 查找最近公共祖先

```js
function findCommonAncestor(node1, node2) {
  const ancestors = new Set();
  
  // 收集 node1 的所有祖先
  let current = node1;
  while (current) {
    ancestors.add(current);
    current = current.parentNode;
  }
  
  // 查找 node2 的祖先中第一个在 ancestors 中的节点
  current = node2;
  while (current) {
    if (ancestors.has(current)) return current;
    current = current.parentNode;
  }
  
  return null;
}

// 原生方法（现代浏览器）
// node1.compareDocumentPosition(node2)
```

---

## 5. 懒加载/无限滚动

```js
// IntersectionObserver 实现懒加载
function lazyLoad(selector) {
  const observer = new IntersectionObserver((entries) => {
    entries.forEach(entry => {
      if (entry.isIntersecting) {
        const img = entry.target;
        img.src = img.dataset.src;
        observer.unobserve(img);
      }
    });
  });
  
  document.querySelectorAll(selector).forEach(img => observer.observe(img));
}

// 无限滚动
function infiniteScroll(container, loadMore) {
  const observer = new IntersectionObserver(([entry]) => {
    if (entry.isIntersecting) loadMore();
  });
  
  // 监听底部占位元素
  const sentinel = document.createElement('div');
  container.appendChild(sentinel);
  observer.observe(sentinel);
}
```

---

## 6. LRU 缓存

```js
class LRUCache {
  constructor(capacity) {
    this.capacity = capacity;
    this.cache = new Map();
  }
  
  get(key) {
    if (!this.cache.has(key)) return -1;
    const value = this.cache.get(key);
    this.cache.delete(key);
    this.cache.set(key, value);
    return value;
  }
  
  put(key, value) {
    if (this.cache.has(key)) this.cache.delete(key);
    this.cache.set(key, value);
    if (this.cache.size > this.capacity) {
      this.cache.delete(this.cache.keys().next().value);
    }
  }
}
```

---

## 7. 大数相加

```js
function addBigNumbers(a, b) {
  const maxLen = Math.max(a.length, b.length);
  a = a.padStart(maxLen, '0');
  b = b.padStart(maxLen, '0');
  
  let carry = 0, result = '';
  for (let i = maxLen - 1; i >= 0; i--) {
    const sum = +a[i] + +b[i] + carry;
    result = (sum % 10) + result;
    carry = Math.floor(sum / 10);
  }
  
  return carry ? carry + result : result;
}

// 示例: addBigNumbers("12345678901234567890", "98765432109876543210")
```