This is one of those questions where knowing the terminology is necessary. In-order traversal visits the left child nodes first, then the root, followed by the right child (remember, it's called in-order because the current node's value is read in-between the left and right child nodes).

Recall the following ordering types:

Inorder traversal

1. First, visit all the nodes in the left subtree
2. Then the root node
3. Visit all the nodes in the right subtree

When to use? With a BST, the in order traversal will give the elements in ascending order, useful for problems where order matters.

1// pseudo-logic of the inorder function:
2inorder(root.left)
3display(root.data)
4inorder(root.right)

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function inorderTraversal(root) {

  let res = [];

  helper(root, res);

  return res;

}

​

function helper(root, res) {

  if (!root) {

    return res;

  }

  helper(root.left, res);

  res.push(root.val);

  helper(root.right, res);

  return res;

}

​

const root = {

  val: 1,

  right: {

    val: 2,

    left: {

      val: 3

    }

  }

};

​

console.log(inorderTraversal(root));