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9. Binary Tree Inorder Traversal

easyAsked at Box

Traverse a binary tree in-order — Box uses this pattern when serializing folder trees in sorted order for the file browser.

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

Problem

Given the root of a binary tree, return the inorder traversal of its nodes' values.

Constraints

  • Number of nodes in [0, 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

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 lefts until null, pop and visit, then move right. Avoids recursion depth.

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:

Box-specific tips

Box prefers the iterative stack approach — they're sensitive to recursion depth because folder trees in enterprise tenants can nest 50+ levels deep.

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Output

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