Write a Program to Find Node Values Using Binary Tree Inorder Traversal

# 94. Binary Tree Inorder Traversal

Given the root of a binary tree, return the inorder traversal of its nodes’ values.

### Example 1:
```
Input: root = [1,null,2,3]
Output: [1,3,2]
```

### Example 2:
```
Input: root = []
Output: []
 ```

Constraints:
```
The number of nodes in the tree is in the range [0, 100].
-100 <= Node.val <= 100
```

/**
 * 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) {
    const out = [];
    const traverse = (node) => {
        if (!node) {
            return;
        }

        traverse(node.left);
        out.push(node.val);
        traverse(node.right);
    } 
    traverse(root);
    return out;   
};