#### 原题说明

Given the root of a binary tree, find the maximum value `V`

for which there exists different nodes `A`

and `B`

where `V = |A.val - B.val|`

and `A`

is an ancestor of `B`

.

(A node `A`

is an ancestor of `B`

if either: any child of `A`

is equal to `B`

, or any child of `A`

is an ancestor of `B`

.)

**Example 1:**

Input:`[8,3,10,1,6,null,14,null,null,4,7,13]`

Output:`7`

Explanation:

We have various ancestor-node differences, some of which are given below :

|8 - 3| = 5

|3 - 7| = 4

|8 - 1| = 7

|10 - 13| = 3

Among all possible differences, the maximum value of 7 is obtained by |8 - 1| = 7.

**Note:**

- The number of nodes in the tree is between
`2`

and`5000`

. - Each node will have value between
`0`

and`100000`

.