Given a binary tree, determine if it is height-balanced.

isBalanced.js

/**
 * Definition for a binary tree node.
 * function TreeNode(val, left, right) {
 *     this.val = (val===undefined ? 0 : val)
 *     this.left = (left===undefined ? null : left)
 *     this.right = (right===undefined ? null : right)
 * }
 */
/**
 * @param {TreeNode} root
 * @return {boolean}
 */
// Determine whether a binary tree is height-balanced.
// A tree is balanced if, at every node, the heights of the left and right
// subtrees differ by no more than 1.
var isBalanced = function(root) {
    // If getHeight returns -1, the tree is unbalanced.
    return getHeight(root) !== -1;

    /**
     * Recursively compute the height of a subtree.
     * Returns:
     *   - a non-negative integer = height of the subtree
     *   - -1 if the subtree is already known to be unbalanced
     */
    function getHeight(node) {
        // An empty subtree has height 0.
        if (!node) {
            return 0;
        }

        // Recursively compute heights of left and right subtrees.
        const leftHeight = getHeight(node.left);
        const rightHeight = getHeight(node.right);

        // If either subtree is unbalanced, or the height difference is > 1,
        // propagate the failure upward by returning -1.
        if (
            leftHeight === -1 ||
            rightHeight === -1 ||
            Math.abs(leftHeight - rightHeight) > 1
        ) {
            return -1;
        }

        // Otherwise, return the height of this subtree.
        return 1 + Math.max(leftHeight, rightHeight);
    }
};