Solution.cs

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left;
 *     public TreeNode right;
 *     public TreeNode(int val=0, TreeNode left=null, TreeNode right=null) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
public class Solution {
    public int CountNodes(TreeNode root) {
        if (root == null) {
            return 0;
        }
        int left = depth(root.left);
        int right = depth(root.right);
        if (left == right) {
            return (1 << left) + CountNodes(root.right);
        }
        return (1 << right) + CountNodes(root.left);
    }

    private int depth(TreeNode root) {
        int d = 0;
        for (; root != null; root = root.left) {
            ++d;
        }
        return d;
    }
}