連結
給你一棵 完全二叉樹 的根節點 root ，求出該樹的節點個數。

完全二叉樹 的定義如下：在完全二叉樹中，除了最底層節點可能沒填滿外，其餘每層節點數都達到最大值，並且最下面一層的節點都集中在該層最左邊的若干位置。若最底層為第 h 層，則該層包含 1~2h個節點。

class Solution {

    private int getDeep(TreeNode root) {
        int ret = 0;
        while (root != null) {
            ret++;
            root = root.left;
        }
        return ret;
    }

    private int solve(TreeNode root, int deep) {
        if (root == null) {
            return 0;
        }
        int leftDeep = deep - 1;
        int rightDeep = getDeep(root.right);

        if (leftDeep == rightDeep) {
            return (1 << leftDeep) + solve(root.right, rightDeep);
        } else {
            return (1 << rightDeep) + solve(root.left, leftDeep);
        }
    }

    public int countNodes(TreeNode root) {
        if (root == null) {
            return 0;
        }

        int deep = getDeep(root);
        return solve(root, deep);
    }
}


class TreeNode {
    int val;
    TreeNode left;
    TreeNode right;

    TreeNode() {
    }

    TreeNode(int val) {
        this.val = val;
    }

    TreeNode(int val, TreeNode left, TreeNode right) {
        this.val = val;
        this.left = left;
        this.right = right;
    }
}