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9. Binary Tree Inorder Traversal

easyAsked at Ola

Return the inorder traversal of a binary tree's node values.

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 visits left subtree, node, then right subtree.

Constraints

  • The number of nodes is 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. Recursion

Recursively traverse left, append node, then right.

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

Tradeoff:

2. Iterative stack

Push left spine onto a stack; pop, record, then walk right child. Same O(n) but explicit stack.

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:

Ola-specific tips

Ola interviewers ask the iterative version to see if you can flatten a recursive walk for a deep zone-tree without blowing the stack on prod traffic.

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Output

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