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# 1530. Number of Good Leaf Nodes Pairs

<https://leetcode.com/problems/number-of-good-leaf-nodes-pairs>

## Description

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:**

![](https://assets.leetcode.com/uploads/2020/07/09/e1.jpg)

```

**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:**

![](https://assets.leetcode.com/uploads/2020/07/09/e2.jpg)

```

**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].
```

**Example 4:**

```

**Input:** root = [100], distance = 1
**Output:** 0
```

**Example 5:**

```

**Input:** root = [1,1,1], distance = 2
**Output:** 1
```

**Constraints:**

* The number of nodes in the `tree` is in the range `[1, 2^10].`
* Each node's value is between `[1, 100]`.
* `1 <= distance <= 10`

## ac

```java
```