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30. Invert Binary Tree

easyAsked at Plaid

Invert (mirror) a binary tree. Plaid asks this as a recursion warm-up before harder category-tree transformations.

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

Source citations

Public interview reports confirming this problem appears in Plaid loops.

  • Glassdoor (2025)— Plaid intro problem.
  • LeetCode Discuss (2026)— Plaid OA warm-up.

Problem

Given the root of a binary tree, invert the tree, and return its root. Inverting means swapping every left subtree with its corresponding right subtree.

Constraints

  • The number of nodes in the tree is in the range [0, 100].
  • -100 <= Node.val <= 100

Examples

Example 1

Input
root = [4,2,7,1,3,6,9]
Output
[4,7,2,9,6,3,1]

Example 2

Input
root = []
Output
[]

Approaches

1. Iterative BFS swap

BFS through the tree; at each node, swap children.

Time
O(n)
Space
O(w)
function invertTree(root) {
  if (!root) return null;
  const q = [root];
  while (q.length) {
    const n = q.shift();
    [n.left, n.right] = [n.right, n.left];
    if (n.left) q.push(n.left);
    if (n.right) q.push(n.right);
  }
  return root;
}

Tradeoff: Avoids recursion stack. shift() is O(n) — use an index pointer or a deque in production.

2. Recursive

Swap children, recurse on each.

Time
O(n)
Space
O(h)
function invertTree(root) {
  if (!root) return null;
  [root.left, root.right] = [invertTree(root.right), invertTree(root.left)];
  return root;
}

Tradeoff: One-line core. The destructuring assignment makes the swap explicit.

Plaid-specific tips

Plaid uses this as a recursion sanity check. Bonus signal: ask whether the input may be huge — if yes, default to the iterative version. Mention that the same pattern shows up when mirroring a category-tree for a left-handed display layout.

Common mistakes

  • Swapping before recursing instead of using destructuring — works but easier to introduce bugs.
  • Returning a new tree — the problem allows in-place mutation, which is cheaper.
  • Forgetting the empty-tree base case.

Follow-up questions

An interviewer at Plaid may pivot to one of these next:

  • Detect if a tree is symmetric (LC 101) — closely related.
  • Invert only one level of the tree.
  • Same problem on N-ary trees.

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Output

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FAQ

Recursive or iterative — which does Plaid prefer?

Either is fine for a 100-node tree. For 10M-node category trees, iterative avoids stack overflow.

Why does destructuring help?

It does the swap in one statement, eliminating the need for a temporary variable and a wrong-order bug.

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