Update 0222.完全二叉树的节点个数.md

增加完全二叉树写法2，更易理解且减少非完全二叉树节点的迭代次数

problems/0222.完全二叉树的节点个数.md

@@ -379,6 +379,20 @@ class Solution:
             return (2 << leftDepth) - 1 #注意(2<<1) 相当于2^2，所以leftDepth初始为0
         return self.countNodes(root.left) + self.countNodes(root.right) + 1
 ```
+完全二叉树写法2
+```python
+class Solution: # 利用完全二叉树特性
+    def countNodes(self, root: TreeNode) -> int:
+        if not root: return 0
+        count = 1
+        left = root.left; right = root.right
+        while left and right:
+            count+=1
+            left = left.left; right = right.right
+        if not left and not right: # 如果同时到底说明是满二叉树，反之则不是
+            return 2**count-1
+        return 1+self.countNodes(root.left)+self.countNodes(root.right)  
+```
 
 ## Go