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Java-based LeetCode algorithm problem solutions, regularly updated

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1609\. Even Odd Tree

Medium

A binary tree is named **Even-Odd** if it meets the following conditions:

*   The root of the binary tree is at level index `0`, its children are at level index `1`, their children are at level index `2`, etc.
*   For every **even-indexed** level, all nodes at the level have **odd** integer values in **strictly increasing** order (from left to right).
*   For every **odd-indexed** level, all nodes at the level have **even** integer values in **strictly decreasing** order (from left to right).

Given the `root` of a binary tree, _return_ `true` _if the binary tree is **Even-Odd**, otherwise return_ `false`_._

**Example 1:**

![](https://assets.leetcode.com/uploads/2020/09/15/sample_1_1966.png)

**Input:** root = [1,10,4,3,null,7,9,12,8,6,null,null,2]

**Output:** true

**Explanation:** The node values on each level are: 

Level 0: [1] 

Level 1: [10,4] 

Level 2: [3,7,9]

Level 3: [12,8,6,2] 

Since levels 0 and 2 are all odd and increasing and levels 1 and 3 are all even and decreasing, the tree is Even-Odd.

**Example 2:**

![](https://assets.leetcode.com/uploads/2020/09/15/sample_2_1966.png)

**Input:** root = [5,4,2,3,3,7]

**Output:** false

**Explanation:** The node values on each level are:

Level 0: [5]

Level 1: [4,2] 

Level 2: [3,3,7] 

Node values in level 2 must be in strictly increasing order, so the tree is not Even-Odd.

**Example 3:**

![](https://assets.leetcode.com/uploads/2020/09/22/sample_1_333_1966.png)

**Input:** root = [5,9,1,3,5,7]

**Output:** false

**Explanation:** Node values in the level 1 should be even integers.

**Constraints:**

*   The number of nodes in the tree is in the range [1, 10⁵].
*   1 <= Node.val <= 10⁶