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‎Problemset/0222-Count Complete Tree Nodes/README.md

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# 222. Count Complete Tree Nodes
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Given the `root` of a **complete** binary tree, return the number of the nodes in the tree.
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According to [**Wikipedia**](http://en.wikipedia.org/wiki/Binary_tree#Types_of_binary_trees), every level, except possibly the last, is completely filled in a complete binary tree, and all nodes in the last level are as far left as possible. It can have between `1` and <code>2<sup>h</sup></code> nodes inclusive at the last level `h`.
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Design an algorithm that runs in less than `O(n)` time complexity.
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#### Example 1:
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![](https://assets.leetcode.com/uploads/2021/01/14/complete.jpg)
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<pre>
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<strong>Input:</strong> root = [1,2,3,4,5,6]
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<strong>Output:</strong> 6
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</pre>
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#### Example 2:
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<pre>
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<strong>Input:</strong> root = []
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<strong>Output:</strong> 0
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</pre>
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#### Example 3:
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<pre>
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<strong>Input:</strong> root = [1]
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<strong>Output:</strong> 1
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</pre>
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#### Constraints:
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* The number of nodes in the tree is in the range <code>[0, 5 * 10<sup>4</sup>]</code>.
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* <code>0 <= Node.val <= 5 * 10<sup>4</sup></code>
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* The tree is guaranteed to be **complete**.
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## Solutions (Python)
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### 1. Solution
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```Python
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# Definition for a binary tree node.
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# class TreeNode:
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# def __init__(self, val=0, left=None, right=None):
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# self.val = val
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# self.left = left
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# self.right = right
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class Solution:
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def countNodes(self, root: Optional[TreeNode]) -> int:
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low = 0
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high = 1
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curr = root
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root = TreeNode(left=TreeNode(), right=root)
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while curr is not None:
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low = (low << 1) + 1
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high = (high << 1) + 1
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curr = curr.right
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while low < high:
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mid = (low + high) // 2
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curr = root
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flag = True
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for bit in bin(mid)[2:]:
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if bit == '0':
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curr = curr.left
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else:
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curr = curr.right
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if curr is None:
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flag = False
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break
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if flag:
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low = mid + 1
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else:
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high = mid
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return low - 1
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```

‎Problemset/0222-Count Complete Tree Nodes/README_CN.md

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# 222. 完全二叉树的节点个数
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给你一棵 **完全二叉树** 的根节点 `root` ，求出该树的节点个数。
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[完全二叉树](https://baike.baidu.com/item/%E5%AE%8C%E5%85%A8%E4%BA%8C%E5%8F%89%E6%A0%91/7773232?fr=aladdin) 的定义如下：在完全二叉树中，除了最底层节点可能没填满外，其余每层节点数都达到最大值，并且最下面一层的节点都集中在该层最左边的若干位置。若最底层为第 `h` 层，则该层包含 <code>1~ 2<sup>h</sup></code> 个节点。
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#### 示例 1:
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![](https://assets.leetcode.com/uploads/2021/01/14/complete.jpg)
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<pre>
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<strong>输入:</strong> root = [1,2,3,4,5,6]
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<strong>输出:</strong> 6
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</pre>
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#### 示例 2:
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<pre>
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<strong>输入:</strong> root = []
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<strong>输出:</strong> 0
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</pre>
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#### 示例 3:
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<pre>
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<strong>输入:</strong> root = [1]
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<strong>输出:</strong> 1
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</pre>
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#### 提示:
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* 树中节点的数目范围是<code>[0, 5 * 10<sup>4</sup>]</code>
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* <code>0 <= Node.val <= 5 * 10<sup>4</sup></code>
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* 题目数据保证输入的树是 **完全二叉树**
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**进阶：**遍历树来统计节点是一种时间复杂度为 `O(n)` 的简单解决方案。你可以设计一个更快的算法吗？
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## 题解 (Python)
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### 1. 题解
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```Python
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# Definition for a binary tree node.
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# class TreeNode:
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# def __init__(self, val=0, left=None, right=None):
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# self.val = val
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# self.left = left
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# self.right = right
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class Solution:
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def countNodes(self, root: Optional[TreeNode]) -> int:
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low = 0
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high = 1
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curr = root
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root = TreeNode(left=TreeNode(), right=root)
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while curr is not None:
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low = (low << 1) + 1
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high = (high << 1) + 1
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curr = curr.right
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while low < high:
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mid = (low + high) // 2
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curr = root
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flag = True
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for bit in bin(mid)[2:]:
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if bit == '0':
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curr = curr.left
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else:
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curr = curr.right
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if curr is None:
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flag = False
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break
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if flag:
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low = mid + 1
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else:
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high = mid
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return low - 1
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```

‎Problemset/0222-Count Complete Tree Nodes/Solution.py

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# Definition for a binary tree node.
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# class TreeNode:
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# def __init__(self, val=0, left=None, right=None):
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# self.val = val
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# self.left = left
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# self.right = right
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class Solution:
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def countNodes(self, root: Optional[TreeNode]) -> int:
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low = 0
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high = 1
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curr = root
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root = TreeNode(left=TreeNode(), right=root)
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while curr is not None:
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low = (low << 1) + 1
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high = (high << 1) + 1
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curr = curr.right
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while low < high:
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mid = (low + high) // 2
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curr = root
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flag = True
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for bit in bin(mid)[2:]:
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if bit == '0':
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curr = curr.left
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else:
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curr = curr.right
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if curr is None:
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flag = False
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break
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if flag:
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low = mid + 1
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else:
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high = mid
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return low - 1

‎README.md

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[219][219l] |[Contains Duplicate II][219] |![rs]
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[220][220l] |[Contains Duplicate III][220] |![rs]
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[221][221l] |[Maximal Square][221] |![rs]
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[222][222l] |[Count Complete Tree Nodes][222] |![py]
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[223][223l] |[Rectangle Area][223] |![rb]&nbsp;&nbsp;![rs]
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[225][225l] |[Implement Stack using Queues][225] |![rs]
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[226][226l] |[Invert Binary Tree][226] |![py]
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[219]:Problemset/0219-Contains%20Duplicate%20II/README.md#219-contains-duplicate-ii
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[220]:Problemset/0220-Contains%20Duplicate%20III/README.md#220-contains-duplicate-iii
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[221]:Problemset/0221-Maximal%20Square/README.md#221-maximal-square
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[222]:Problemset/0222-Count%20Complete%20Tree%20Nodes/README.md#222-count-complete-tree-nodes
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[223]:Problemset/0223-Rectangle%20Area/README.md#223-rectangle-area
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[225]:Problemset/0225-Implement%20Stack%20using%20Queues/README.md#225-implement-stack-using-queues
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[226]:Problemset/0226-Invert%20Binary%20Tree/README.md#226-invert-binary-tree
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[219l]:https://leetcode.com/problems/contains-duplicate-ii/
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[220l]:https://leetcode.com/problems/contains-duplicate-iii/
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[221l]:https://leetcode.com/problems/maximal-square/
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[222l]:https://leetcode.com/problems/count-complete-tree-nodes/
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[223l]:https://leetcode.com/problems/rectangle-area/
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[225l]:https://leetcode.com/problems/implement-stack-using-queues/
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[226l]:https://leetcode.com/problems/invert-binary-tree/

‎README_CN.md

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[219][219l] |[存在重复元素 II][219] |![rs]
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[220][220l] |[存在重复元素 III][220] |![rs]
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[221][221l] |[最大正方形][221] |![rs]
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[222][222l] |[完全二叉树的节点个数][222] |![py]
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[223][223l] |[矩形面积][223] |![rb]&nbsp;&nbsp;![rs]
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[225][225l] |[用队列实现栈][225] |![rs]
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[226][226l] |[翻转二叉树][226] |![py]
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[219]:Problemset/0219-Contains%20Duplicate%20II/README_CN.md#219-存在重复元素-ii
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[220]:Problemset/0220-Contains%20Duplicate%20III/README_CN.md#220-存在重复元素-iii
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[221]:Problemset/0221-Maximal%20Square/README_CN.md#221-最大正方形
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[222]:Problemset/0222-Count%20Complete%20Tree%20Nodes/README_CN.md#222-完全二叉树的节点个数
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[223]:Problemset/0223-Rectangle%20Area/README_CN.md#223-矩形面积
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[225]:Problemset/0225-Implement%20Stack%20using%20Queues/README_CN.md#225-用队列实现栈
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[226]:Problemset/0226-Invert%20Binary%20Tree/README_CN.md#226-翻转二叉树
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[219l]:https://leetcode.cn/problems/contains-duplicate-ii/
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[220l]:https://leetcode.cn/problems/contains-duplicate-iii/
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[221l]:https://leetcode.cn/problems/maximal-square/
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[222l]:https://leetcode.cn/problems/count-complete-tree-nodes/
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[223l]:https://leetcode.cn/problems/rectangle-area/
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[225l]:https://leetcode.cn/problems/implement-stack-using-queues/
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[226l]:https://leetcode.cn/problems/invert-binary-tree/

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