isBalanced = function(root) {
    const isBaseCase = root === null;
    if (isBaseCase) return true;
    if (!isAcceptableHeight(root)) return false;
    if (!isChildBalanced(root)) return false;

    return true;
}
https://leetcode.com/problems/balanced-binary-tree/
TIme O(N) | Space O(H)
@param {TreeNode} root
@return {boolean}