1. var countNodes = function(root) {
    2.     if (root == null) return 0;
    3.     let l = root, r = root;
    4.     let lh = 0, rh = 0;
    5.     while (l != null) {
    6.       l = l.left;
    7.       lh++;
    8.     }
    9.     while (r != null) {
    10.       r = r.right;
    11.       rh++;
    12.     }
    13.     if (lh === rh) {
    14.       return Math.pow(2, lh) - 1;
    15.     }
    16.     return 1 + countNodes(root.left) + countNodes(root.right);
    17. };