complete

Definition of a complete binary tree from ​​Wikipedia​​​:
In a complete binary tree every level, except possibly the last, is completely filled, and all nodes in the last level are as far left as possible. It can have between 1 and 2h

1.先向最左和最右走，看是否是满二叉树；如果是可以直接算法节点数目；

2.否则，对根的左右节点递归上述过程。

/**
 * 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) {
    int hl = 0, hr = 0;
    TreeNode* l = root, *r = root;
    while (l){
      l = l->left;
      hl++;
    }
    while (r){
      r = r->right;
      hr++;
    }
    if (hl == hr) return (1 << hl) - 1;
    else{
      return 1 + countNodes(root->left) + countNodes(root->right);
    }
  }
};