[LC] 222. Count Complete Tree Nodes

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

Note:

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.

Example:

Input: 
    1
   / 
  2   3
 /   /
4  5 6

Output: 6

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