Given acompletebinary 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 2hnodes inclusive at the last level h.

Solution: 对于full tree，node总数为2^height - 1。对于complete tree，所以先在子树找到full tree，直接用高度计算node总数

    public int countNodes(TreeNode root) {
        if (root == null)
            return 0;
        TreeNode left = root, right = root;
        int height = 0;
        while (left != null && right != null) {
            left = left.left;
            right = right.right;
            height++;
        }
        //此时为full tree
        if (left == null && right == null) {
            return (1 << height) - 1;
        }
        //不full时用普通方法
        return 1 + countNodes(root.left) + countNodes(root.right);
    }