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9. Binary Tree Inorder Traversal

easyAsked at TripAdvisor

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. Solve both recursively and iteratively if asked.

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

Left subtree, node, right subtree.

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

Tradeoff:

2. Iterative with stack

Push left chain, pop and visit, then move right. Avoids recursion limits.

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:

TripAdvisor-specific tips

TripAdvisor uses tree-walk warm-ups to gauge how cleanly you handle category/location hierarchies before moving to geo-sharding problems.

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