Count Complete Tree Nodes

Posted 2020-08-25 北叶青藤

Given a complete binary 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 2h nodes inclusive at the last level h.

 1 /**
 2  * Definition for a binary tree node.
 3  * public class TreeNode {
 4  *     int val;
 5  *     TreeNode left;
 6  *     TreeNode right;
 7  *     TreeNode(int x) { val = x; }
 8  * }
 9  */
10 public class Solution {
11     public int countNodes(TreeNode root) {
12     
13         int leftDepth = leftDepth(root);
14         int rightDepth = rightDepth(root);
15     
16         if (leftDepth == rightDepth) {
17             return (1 << leftDepth) - 1;
18         }
19         return 1 + countNodes(root.left) + countNodes(root.right);
20     
21     }
22     
23     private int rightDepth(TreeNode root) {
24         int depth = 0;
25         while (root != null) {
26             root = root.right;
27             depth++;
28         }
29         return depth;
30     }
31     
32     private int leftDepth(TreeNode root) {
33         int depth = 0;
34         while (root != null) {
35             root = root.left;
36             depth++;
37         }
38         return depth;
39     }
40 }