Balanced Binary Tree: the idea to remember.

The core insight

Return a height or a negative sentinel when any subtree is unbalanced.

Why it works

Each return certifies the entire subtree, not just its root. A negative result propagates immediately.

The mistake to avoid

Checking the root alone misses an unbalanced descendant. Recomputing heights at every node can take O(n²).

Commented Java solution

Use java.util.* imports and a surrounding solution class. Tree snippets assume the standard TreeNode definition; linked lists use ListNode and Clone Graph uses a Node with val and neighbors; Copy Random List uses Node with val, next and random. Read the input assumptions above.

boolean isBalanced(TreeNode root) { return balancedHeight(root) != -1; }
int balancedHeight(TreeNode node) {
    if (node == null) return 0;
    int l = balancedHeight(node.left);
    if (l == -1) return -1;
    int r = balancedHeight(node.right);
    if (r == -1 || Math.abs(l - r) > 1) return -1;
    return 1 + Math.max(l, r);
}

Make the next connection