/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
    int countNodes(TreeNode* root) {
       if(!root)return 0;
       TreeNode *l=root,*r=root;
       int hl=0,hr=0;
       while(l){hl++;l=l->left;}
       while(r){hr++;r=r->right;}
       if(hl==hr)return (1<<hl)-1;
       return 1+countNodes(root->left)+countNodes(root->right);
    }
};