https://leetcode.com/problems/count-complete-tree-nodes/#/description

http://www.cnblogs.com/EdwardLiu/p/5058570.html

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public int countNodes(TreeNode root) {
        if (root == null) return 0;
        int leftHeight = countLeft(root);
        int rightHeight = countRight(root);
        if (leftHeight == rightHeight) { //perfect binary tree
            return (1<<leftHeight)-1;
        }
        else return countNodes(root.left) + countNodes(root.right) + 1;
    }
    
    public int countLeft(TreeNode root) {
        int res = 0;
        while (root != null) {
            res++;
            root = root.left;
        }
        return res;
    }
    
    public int countRight(TreeNode root) {
        int res = 0;
        while (root != null) {
            res++;
            root = root.right;
        }
        return res;
    }
    
}

Count Complete Tree Nodes