Count Complete Tree Nodes

Given a complete binary tree, count the number of nodes.

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 nodes inclusive at the last level h.

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思路： Using height to make it to compare countLeft with countRight. Then, recursion to resolve the subquestion.

/**
 * 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 = countHelper(root.left);
        int rightHeight = countHelper(root.right);
        if(leftHeight == rightHeight)
            return (1 << leftHeight) + countNodes(root.right);
        return (1 << rightHeight) + countNodes(root.left);
    }
    
    private int countHelper(TreeNode root){
        int height = 0;
        while(root != null){
            height++;
            root = root.left;
        }
        return height;
    }
}