How do you check if a binary tree is balanced?

A balanced binary tree has the height difference between left and right subtrees of every node at most 1. The optimal O(n) solution uses a bottom-up approach: recursively compute height while checking balance, returning -1 if imbalanced. For each node, get heights of left and right subtrees; if either is -1 or their difference exceeds 1, return -1; otherwise return max(left, right) + 1.