1530. Number of Good Leaf Nodes Pairs

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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 = 3Output:1Explanation: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 = 3Output:2Explanation: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 = 3Output:1Explanation:The only good pair is [2,5].

**Constraints:**

- The number of nodes in the
`tree`

is in the range`[1, 2`

^{10}]. `1 <= Node.val <= 100`

`1 <= distance <= 10`

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