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9. Binary Tree Inorder Traversal

easyAsked at Indeed

Return the inorder traversal of a binary tree — Indeed's tree-walk warmup that maps to traversing taxonomy nodes in a job-category tree.

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

Problem

Given the root of a binary tree, return the inorder traversal of its nodes' values. The expected output visits left subtree, node, then right subtree.

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

Standard recursive left/root/right walk.

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

Tradeoff:

2. Iterative stack

Push lefts onto a stack, pop when bottoming out, then move right.

Time
O(n)
Space
O(h)
function inorder(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:

Indeed-specific tips

Indeed prefers the iterative stack version on phone screens — they fear recursion blowing the stack on deep job-category subtrees.

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Output

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