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9. Binary Tree Inorder Traversal

easyAsked at Monzo

Return the inorder traversal of a tree (useful for sorted category 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. Inorder = left subtree, node, right subtree.

Constraints

  • Number of nodes is in [0, 100]

Examples

Example 1

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

Example 2

Input
root = []
Output
[]

Approaches

1. Recursive

Recurse left, push node, recurse right.

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 chain, pop and emit, then descend right.

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:

Monzo-specific tips

Monzo expects you to show both versions and know why iterative wins on deeply unbalanced trees (their merchant-category hierarchy can be thousands deep).

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Output

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