
# Binary Tree Inorder Traversal

## Description

[doc](https://leetcode.com/problems/binary-tree-inorder-traversal/)

## Solutions

### First Idea

```javascript
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function (root) {
  if (!root) {
    return []
  }

  if (!root.left && !root.right) {
    return [root.val]
  }

  return [
    ...inorderTraversal(root.left),
    root.val,
    ...inorderTraversal(root.right),
  ]
}
```

- Time Complexity: O(n)
- Space Complexity: O(n)

### Refined one

```javascript
/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {number[]}
 */
var inorderTraversal = function (root) {
  if (!root) {
    return []
  }

  return [
    ...inorderTraversal(root.left),
    root.val,
    ...inorderTraversal(root.right),
  ]
}
```

- Time Complexity: O(n)
- Space Complexity: O(n)
