LeetCode number of complete binary tree node [222]

Title:
given a complete binary tree, obtains the number of nodes of the tree.

Description:

Complete binary tree is defined as follows: in the binary tree, except the bottom node may not be filled, the remaining maximum number of nodes each, and the bottom layer of nodes are concentrated in a plurality of layers of the leftmost position. If the bottom of the h layer, the layer comprising the nodes 1 ~ 2h.

Example:

Input:

Output: 6

/**

• There are two possibilities:
• 1, hl = hr left subtree full, direct calculation of the left subtree and recursively right subtree
• 2, hl> hr right subtree of the full tree, than the left subtree low level, direct calculation of the right subtree node, then the left subtree recursively
• The time complexity of O (h ^ 2) h is the height of the tree
  * /

	public int countNodes(TreeNode root) {
        if (root == null)
            return 0;
        int hl = depth(root.left);
        int hr = depth(root.right);
        if (hl == hr) {
            // 注：1 为根节点  (1 << hl) - 1 为左子树节点个数 countNodes(root.right)为右子树节点个数
            return 1 + ((1 << hl) - 1) + countNodes(root.right);
        } else {
            return 1 + ((1 << hr) - 1) + countNodes(root.left);
        }
    }

    private int depth(TreeNode node) {
        int d = 0;
        while (node != null) {
            node = node.left;
            d++;
        }
        return d;
    }