---
id: 587d8251367417b2b2512c64
title: 在链接列表中搜索
challengeType: 1
videoUrl: ''
dashedName: search-within-a-linked-list
---

# --description--

让我们为链表类添加一些更有用的方法。如果我们可以判断我们的列表是否为空，那么它是否有用，就像我们的`Stack`和`Queue`类一样？我们还应该能够在链表中找到特定元素。遍历数据结构是你想要进行大量练习的东西！让我们创建一个`indexOf`方法，该方法将`element`作为参数，并在链表中返回该元素的`index` 。如果在链接列表中找不到该元素，则返回`-1` 。让我们实现一个相反的方法：一个`elementAt`方法，它将`index`作为参数并返回给定`index`处的`element` 。如果未找到任何`element` ，则返回`undefined` 。

# --instructions--

编写一个检查链表是否为空的`isEmpty`方法，返回给定元素`index`的`indexOf`方法，以及返回给定`index.`处`element`的`elementAt` `index.`

# --hints--

您的`LinkedList`类应该有一个`indexOf`方法。

```js
assert(
  (function () {
    var test = new LinkedList();
    return typeof test.indexOf === 'function';
  })()
);
```

您的`LinkedList`类应该有一个`elementAt`方法。

```js
assert(
  (function () {
    var test = new LinkedList();
    return typeof test.elementAt === 'function';
  })()
);
```

您的`size`方法应返回链表的长度

```js
assert(
  (function () {
    var test = new LinkedList();
    test.add('cat');
    test.add('dog');
    test.add('kitten');
    return test.size() === 3;
  })()
);
```

`indexOf`方法应该返回给定元素的索引。

```js
assert(
  (function () {
    var test = new LinkedList();
    test.add('cat');
    test.add('dog');
    test.add('kitten');
    return test.indexOf('kitten') === 2;
  })()
);
```

您的`elementAt`方法应该返回给定索引处的元素。

```js
assert(
  (function () {
    var test = new LinkedList();
    test.add('cat');
    test.add('dog');
    test.add('kitten');
    return test.elementAt(1) === 'dog';
  })()
);
```

# --seed--

## --seed-contents--

```js
function LinkedList() {
  var length = 0;
  var head = null;

  var Node = function(element){
    this.element = element;
    this.next = null;
  };

  this.size = function() {
    return length;
  };

  this.head = function(){
    return head;
  };

  this.add = function(element){
    var node = new Node(element);
    if(head === null){
        head = node;
    } else {
      var currentNode = head;

      while(currentNode.next){
        currentNode = currentNode.next;
      }

      currentNode.next = node;
    }

    length++;
  };

  this.remove = function(element){
    var currentNode = head;
    var previousNode;
    if(currentNode.element === element){
      head = currentNode.next;
    } else {
      while(currentNode.element !== element) {
        previousNode = currentNode;
        currentNode = currentNode.next;
      }

      previousNode.next = currentNode.next;
    }

    length --;
  };

  // Only change code below this line

  // Only change code above this line
}
```

# --solutions--

```js
function LinkedList() {
  var length = 0;
  var head = null;

  var Node = function(element){
    this.element = element;
    this.next = null;
  };

  this.size = function() {
    return length;
  };

  this.head = function(){
    return head;
  };

  this.add = function(element){
    var node = new Node(element);
    if(head === null){
        head = node;
    } else {
        var currentNode = head;

        while(currentNode.next){
            currentNode  = currentNode.next;
        }

        currentNode.next = node;
    }

    length++;
  };

  this.remove = function(element){
    var currentNode = head;
    var previousNode;
    if(currentNode.element === element){
        head = currentNode.next;
    } else {
        while(currentNode.element !== element) {
            previousNode = currentNode;
            currentNode = currentNode.next;
        }

        previousNode.next = currentNode.next;
    }

    length --;
  };

  this.indexOf = function(element) {
    if (head === null) return -1

    let current = head;
    let index = 0;

    while (current.element !== element && current.next !== null) {
      current = current.next;
      index++
    }

    if (current.element !== element && current.next === null) {
      return -1
    }

    return index;
  }

  this.elementAt = function(index) {
    if (head === null) return undefined;

    let current = head;
    let currentIndex = 0;

    while (currentIndex !== index && current.next !== null) {
      current = current.next;
      currentIndex++
    }

    if (currentIndex !== index && current.next === null) {
      return undefined;
    }

    return current.element;
  }

  this.isEmpty = function() {
    return length === 0;
  }
}
```