public class Solution
extends Object
1530 - Number of Good Leaf Nodes Pairs\.
Medium
You are given the `root` of a binary tree and an integer `distance`. A pair of two different **leaf** nodes of a binary tree is said to be good if the length of **the shortest path** between them is less than or equal to `distance`.
Return _the number of good leaf node pairs_ in the tree.
**Example 1:**
**Input:** root = [1,2,3,null,4], distance = 3
**Output:** 1
**Explanation:** The leaf nodes of the tree are 3 and 4 and the length of the shortest path between them is 3. This is the only good pair.
**Example 2:**
**Input:** root = [1,2,3,4,5,6,7], distance = 3
**Output:** 2
**Explanation:** The good pairs are [4,5] and [6,7] with shortest path = 2. The pair [4,6] is not good because the length of ther shortest path between them is 4.
**Example 3:**
**Input:** root = [7,1,4,6,null,5,3,null,null,null,null,null,2], distance = 3
**Output:** 1
**Explanation:** The only good pair is [2,5].
**Constraints:**
* The number of nodes in the `tree` is in the range [1, 210].
* `1 <= Node.val <= 100`
* `1 <= distance <= 10`