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9. Binary Tree Inorder Traversal

easyAsked at Instacart

In-order traverse a binary tree — Instacart uses this to confirm tree fluency before category-hierarchy walking problems.

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.

Constraints

  • The number of nodes in the tree is in the range [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 DFS

Recurse left, push root, recurse right.

Time
O(n)
Space
O(h)
const res = [];
function dfs(n) {
  if (!n) return;
  dfs(n.left); res.push(n.val); dfs(n.right);
}
dfs(root);
return res;

Tradeoff:

2. Iterative stack

Walk left pushing onto a stack, then pop and pivot right.

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

Tradeoff:

Instacart-specific tips

Instacart's interviewers may follow up with 'walk the aisle-category tree in display order' — be ready to defend why iterative is preferred for deep production trees.

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Output

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