Number of Good Leaf Nodes Pairs
<p>You are given the <code>root</code> of a binary tree and an integer <code>distance</code>. A pair of two different <strong>leaf</strong> nodes of a binary tree is said to be good if the length of <strong>the shortest path</strong> between them is less than or equal to <code>distance</code>.</p>

<p>Return <em>the number of good leaf node pairs</em> in the tree.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1500-1599/1530.Number%20of%20Good%20Leaf%20Nodes%20Pairs/images/e1.jpg" style="width: 250px; height: 250px;" />
<pre>
<strong>Input:</strong> root = [1,2,3,null,4], distance = 3
<strong>Output:</strong> 1
<strong>Explanation:</strong> 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.
</pre>

<p><strong class="example">Example 2:</strong></p>
<img alt="" src="https://fastly.jsdelivr.net/gh/doocs/leetcode@main/solution/1500-1599/1530.Number%20of%20Good%20Leaf%20Nodes%20Pairs/images/e2.jpg" style="width: 250px; height: 182px;" />
<pre>
<strong>Input:</strong> root = [1,2,3,4,5,6,7], distance = 3
<strong>Output:</strong> 2
<strong>Explanation:</strong> 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.
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> root = [7,1,4,6,null,5,3,null,null,null,null,null,2], distance = 3
<strong>Output:</strong> 1
<strong>Explanation:</strong> The only good pair is [2,5].
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the <code>tree</code> is in the range <code>[1, 2<sup>10</sup>].</code></li>
	<li><code>1 &lt;= Node.val &lt;= 100</code></li>
	<li><code>1 &lt;= distance &lt;= 10</code></li>
</ul>
Tree; Depth-First Search; Binary Tree
function countPairs(root: TreeNode | null, distance: number): number {
    const pairs: number[][] = [];

    const dfs = (node: TreeNode | null): number[][] => {
        if (!node) return [];
        if (!node.left && !node.right) return [[node.val, 1]];

        const left = dfs(node.left);
        const right = dfs(node.right);

        for (const [x, dx] of left) {
            for (const [y, dy] of right) {
                if (dx + dy <= distance) {
                    pairs.push([x, y]);
                }
            }
        }

        const res: number[][] = [];
        for (const arr of [left, right]) {
            for (const x of arr) {
                if (++x[1] <= distance) res.push(x);
            }
        }

        return res;
    };

    dfs(root);

    return pairs.length;
}
