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9. Binary Tree Inorder Traversal

easyAsked at Dropbox

Return the inorder traversal of a binary tree; Dropbox uses it to probe iterative-tree fluency for walking nested folder hierarchies.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given the root of a binary tree, return the inorder traversal of its nodes' values (left, root, right). Recursive and iterative solutions are both expected.

Constraints

  • 0 <= nodes <= 100
  • -100 <= Node.val <= 100

Examples

Example 1

Input
root=[1,null,2,3]
Output
[1,3,2]

Example 2

Input
root=[]
Output
[]

Approaches

1. Recursive

Visit left subtree, push root.val, visit right subtree.

Time
O(n)
Space
O(h)
function inorder(node,out=[]) {
  if(!node) return out;
  inorder(node.left,out);
  out.push(node.val);
  inorder(node.right,out);
  return out;
}

Tradeoff:

2. Iterative with stack

Push lefts, pop and emit, then descend right. Equivalent to the explicit call-stack form of recursion.

Time
O(n)
Space
O(h)
function inorderTraversal(root) {
  const out = [], stack = [];
  let cur = root;
  while (cur || stack.length) {
    while (cur) { stack.push(cur); cur = cur.left; }
    cur = stack.pop();
    out.push(cur.val);
    cur = cur.right;
  }
  return out;
}

Tradeoff:

Dropbox-specific tips

Dropbox interviewers often follow up with 'now do it without recursion' — be ready to write the explicit-stack version cleanly, that's where they really grade.

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Output

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