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10. Binary Tree Inorder Traversal

easyAsked at Udemy

Walk a binary tree in inorder — Udemy uses this as a stack-recursion warm-up before harder course-category tree problems.

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

Problem

Given the root of a binary tree, return the inorder (left, root, right) traversal of its node values.

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. Recursion

Recurse left, push the node, recurse right.

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 left chain onto a stack; pop, record value, pivot to right child. Continues until both the stack and pointer are empty.

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:

Udemy-specific tips

Udemy interviewers value candidates who can speak to recursion vs iteration trade-offs in deeply nested course-category trees that may overflow the call stack.

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Output

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