递归

class Solution {
    public int countNodes(TreeNode root) {

        if (root == null){
            return 0;
        }

        return countNodes(root.left) + countNodes(root.right) + 1;
    }
}

/**
 * 时间复杂度 O(n)
 * 空间复杂度 O(logn)
 */

https://leetcode-cn.com/problems/count-complete-tree-nodes/