/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public int countNodes(TreeNode root) {
        if (root == null) {
            return 0;
        }
        if (root.left == null && root.right == null) {
            return 1;
        }

        TreeNode left = root;
        int leftDepth = 0;
        while (left != null) {
            leftDepth++;
            left = left.left;
        }

        TreeNode right = root;
        int rightDepth = 0;
        while (right != null) {
            rightDepth++;
            right = right.right;
        }

        if (leftDepth == rightDepth) {
            return (1 << leftDepth) - 1; //same to Math.pow(2, leftDepth)
        } else {
            return 1 + countNodes(root.left) + countNodes(root.right); // root + count of left nodes+  count of right nodes
        }
    }
}