Skip to main content

9. Binary Tree Inorder Traversal

easyAsked at Chegg

Return in-order values of a binary tree — Chegg uses this to check stack-based traversal on subject-taxonomy trees.

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

Problem

Given the root of a binary tree, return the in-order traversal of its nodes' values (left, root, right).

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 left-self-right recursion.

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 stack

Push lefts onto a stack; pop, visit, then descend the right subtree. Avoids recursion-depth blowups.

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

Tradeoff:

Chegg-specific tips

Chegg interviewers like the iterative version because their subject-taxonomy trees can hit several thousand nested topics and recursive walks blow the JS call stack on lambda runs.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

Practice these live with InterviewChamp.AI

Drill Binary Tree Inorder Traversal and other Chegg interview questions under real-loop conditions with instant feedback on your reasoning, complexity claims, and code.

Practice these live with InterviewChamp.AI →