110. Balanced Binary Tree

Given a binary tree, determine if it is height-balanced: for every node, the heights of its two subtrees differ by at most one.

Return each subtree's height from a post-order DFS, or −1 as soon as any subtree is unbalanced. Each node is visited once.

1function isBalanced(root: TreeNode | null): boolean {
2  function check(n: TreeNode | null): number {
3    if (!n) return 0;
4    const l = check(n.left);
5    const r = check(n.right);
6    if (l === -1 || r === -1 || Math.abs(l - r) > 1) return -1;
7    return 1 + Math.max(l, r);
8  }
9  return check(root) !== -1;
10}

tree

At 1: check the left subtree.

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