Skip to main content

13. Balanced Binary Tree

easyAsked at TripAdvisor

Determine if a binary tree is height-balanced.

By Sam K., Founder, InterviewChamp.AI · Last verified

Problem

Given a binary tree, determine if it is height-balanced. A height-balanced binary tree is defined as one in which the left and right subtrees of every node differ in height by no more than 1.

Constraints

  • 0 <= nodes <= 5000
  • -10^4 <= Node.val <= 10^4

Examples

Example 1

Input
root = [3,9,20,null,null,15,7]
Output
true

Example 2

Input
root = [1,2,2,3,3,null,null,4,4]
Output
false

Approaches

1. Recompute height per node

For each node, compute heights of both subtrees independently.

Time
O(n^2)
Space
O(h)
const height = (n) => n ? 1 + Math.max(height(n.left), height(n.right)) : 0;
const check = (n) => !n || (Math.abs(height(n.left) - height(n.right)) <= 1 && check(n.left) && check(n.right));
return check(root);

Tradeoff:

2. Bottom-up DFS with sentinel

Return -1 from unbalanced subtree so we short-circuit. Each node visited once.

Time
O(n)
Space
O(h)
function isBalanced(root) {
  const dfs = (n) => {
    if (!n) return 0;
    const l = dfs(n.left); if (l === -1) return -1;
    const r = dfs(n.right); if (r === -1) return -1;
    if (Math.abs(l - r) > 1) return -1;
    return 1 + Math.max(l, r);
  };
  return dfs(root) !== -1;
}

Tradeoff:

TripAdvisor-specific tips

TripAdvisor uses balanced-tree checks as a proxy for spotting skewed category trees in tag taxonomies.

Solve it now

Free. No sign-up. Python and JavaScript run instantly in your browser.

Output

Press Run or Cmd+Enter to execute

More TripAdvisor coding interview questions

See all TripAdvisor problems →

Companies that also ask Balanced Binary Tree