<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://assets.leetcode.com/uploads/2020/07/09/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://assets.leetcode.com/uploads/2020/07/09/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>

class Solution {
public:
    unordered_map<TreeNode*, vector<TreeNode*>> adj;
    vector<TreeNode*> leaves;

    void buildGraph(TreeNode* root, TreeNode* parent) {
        if (!root) return;
        if (!root->left && !root->right) {
            leaves.push_back(root);
        }
        if (parent) {
            adj[root].push_back(parent);
            adj[parent].push_back(root);
        }
        if (root->left) buildGraph(root->left, root);
        if (root->right) buildGraph(root->right, root);
    }

    int bfs(TreeNode* start, int distance) {
        unordered_map<TreeNode*, bool> visited;
        queue<pair<TreeNode*, int>> q;
        q.push({start, 0});
        visited[start] = true;
        int count = 0;

        while (!q.empty()) {
            auto [node, dist] = q.front();
            q.pop();
            if (dist > distance) continue;
            if (dist > 0 && !node->left && !node->right) {
                count++;
            }
            for (auto neighbor : adj[node]) {
                if (!visited[neighbor]) {
                    visited[neighbor] = true;
                    q.push({neighbor, dist + 1});
                }
            }
        }
        return count;
    }

    int countPairs(TreeNode* root, int distance) {
        if (!root) return 0;
        buildGraph(root, nullptr);
        int totalPairs = 0;
        for (auto leaf : leaves) {
            totalPairs += bfs(leaf, distance);
        }
        return totalPairs / 2;
    }
};

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