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9. Binary Tree Inorder Traversal

easyAsked at Canva

Return inorder traversal values of a binary tree — Canva uses this to check that you can walk tree structures, which mirror nested layer hierarchies.

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

Problem

Given the root of a binary tree, return the inorder traversal of its node 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

Straightforward DFS, but uses recursion stack.

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 with stack

Push left spine, pop, visit, descend right. Lets you handle deep trees without blowing the call stack.

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

Tradeoff:

Canva-specific tips

Canva favors the iterative version because their layer-tree walker must be safe against deeply nested group hierarchies.

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