freeCodeCamp/curriculum/challenges/chinese/10-coding-interview-prep/data-structures/add-a-new-element-to-a-binary-search-tree.md

---
id: 587d8257367417b2b2512c7b
title: 将新元素添加到二叉搜索树
challengeType: 1
videoUrl: ''
dashedName: add-a-new-element-to-a-binary-search-tree
---

# --description--

现在我们已经了解了基础知识，让我们编写一个更复杂的方法。在此挑战中，我们将创建一个向二叉搜索树添加新值的方法。该方法应该被称为`add` ，它应该接受一个整数值来添加到树中。注意保持二叉搜索树的不变量：每个左子项中的值应小于或等于父值，并且每个右子项中的值应大于或等于父值。在这里，让我们这样做，以便我们的树不能容纳重复的值。如果我们尝试添加已存在的值，则该方法应返回`null` 。否则，如果添加成功，则应返回`undefined` 。提示：树是自然递归的数据结构！

# --hints--

存在`BinarySearchTree`数据结构。

```js
assert(
  (function () {
    var test = false;
    if (typeof BinarySearchTree !== 'undefined') {
      test = new BinarySearchTree();
    }
    return typeof test == 'object';
  })()
);
```

二叉搜索树有一个名为`add`的方法。

```js
assert(
  (function () {
    var test = false;
    if (typeof BinarySearchTree !== 'undefined') {
      test = new BinarySearchTree();
    } else {
      return false;
    }
    return typeof test.add == 'function';
  })()
);
```

add方法根据二叉搜索树规则添加元素。

```js
assert(
  (function () {
    var test = false;
    if (typeof BinarySearchTree !== 'undefined') {
      test = new BinarySearchTree();
    } else {
      return false;
    }
    if (typeof test.add !== 'function') {
      return false;
    }
    test.add(4);
    test.add(1);
    test.add(7);
    test.add(87);
    test.add(34);
    test.add(45);
    test.add(73);
    test.add(8);
    const expectedResult = [1, 4, 7, 8, 34, 45, 73, 87];
    const result = test.inOrder();
    return expectedResult.toString() === result.toString();
  })()
);
```

添加已存在的元素将返回`null`

```js
assert(
  (function () {
    var test = false;
    if (typeof BinarySearchTree !== 'undefined') {
      test = new BinarySearchTree();
    } else {
      return false;
    }
    if (typeof test.add !== 'function') {
      return false;
    }
    test.add(4);
    return test.add(4) == null;
  })()
);
```

# --seed--

## --after-user-code--

```js
BinarySearchTree.prototype = Object.assign(
  BinarySearchTree.prototype,
  {
    inOrder() {
      if (!this.root) {
        return null;
      }
      var result = new Array();
      function traverseInOrder(node) {
        node.left && traverseInOrder(node.left);
        result.push(node.value);
        node.right && traverseInOrder(node.right);
      }
      traverseInOrder(this.root);
      return result;
    }
  }
);
```

## --seed-contents--

```js
var displayTree = tree => console.log(JSON.stringify(tree, null, 2));
function Node(value) {
  this.value = value;
  this.left = null;
  this.right = null;
}
function BinarySearchTree() {
  this.root = null;
  // Only change code below this line
  
  // Only change code above this line
}
```

# --solutions--

```js
function Node(value) {
  this.value = value;
  this.left = null;
  this.right = null;
}
function BinarySearchTree() {
  this.root = null;
  this.add = function(element) {
    let current = this.root;
    if (!current) {
      this.root = new Node(element);
      return;
    } else {
      const searchTree = function(current) {
        if (current.value > element) {
          if (current.left) {
            return searchTree(current.left);
          } else {
            current.left = new Node(element);
            return;
          }
        } else if (current.value < element) {
          if (current.right) {
            return searchTree(current.right);
          } else {
            current.right = new Node(element);
            return;
          }
        } else {
          return null;
        }
      };
      return searchTree(current);
    }
  };
}
```