Problem

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 6Output: 6

Solution

class Solution {    public int countNodes(TreeNode root) {        if (root == null) return 0;        int height = 0;        TreeNode left = root.left, right = root.right;        while (left != null && right != null) {            height++;            left = left.left;            right = right.right;        }        if (left == null && right == null) return (1 << (height+1)) - 1;        else return 1 + countNodes(root.left) + countNodes(root.right);    }}