<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>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    void buildAdjList(TreeNode* root, std::unordered_map<TreeNode*, std::vector<TreeNode*>>& adjList)
    {
        if(root->left != nullptr)
        {
            adjList[root->left].emplace_back(root);
            adjList[root].emplace_back(root->left);
            buildAdjList(root->left, adjList);
        }

        if(root->right != nullptr)
        {
            adjList[root->right].emplace_back(root);
            adjList[root].emplace_back(root->right);
            buildAdjList(root->right, adjList);
        }

        return;
    }

    int bfs(TreeNode* root, std::unordered_map<TreeNode*, std::vector<TreeNode*>>& adjList,
     std::unordered_set<TreeNode*>& leafNodes, const int distance)
    {
        int returnVal = 0;

        for(TreeNode* leafNode : leafNodes)
        {
            std::queue<std::pair<TreeNode*, TreeNode*>> nodeToParentQ;
            nodeToParentQ.emplace(std::make_pair(leafNode, nullptr));
            int remainingDistance = distance;

            while(nodeToParentQ.empty() == false && remainingDistance > 0)
            {
                --remainingDistance;
                int qSize = nodeToParentQ.size();
                for(int i = 0; i < qSize; i++)
                {
                    TreeNode* node = nodeToParentQ.front().first;
                    TreeNode* parent = nodeToParentQ.front().second;
                    nodeToParentQ.pop();

                    for(TreeNode* adjNode : adjList[node])
                    {
                        if(adjNode == parent) continue;
                        if(leafNodes.count(adjNode))
                        {
                            ++returnVal;
                            continue;
                        }

                        nodeToParentQ.emplace(std::make_pair(adjNode, node));
                    }
                }
            }
        }

        return returnVal / 2;
    }

    int countPairs(TreeNode* root, int distance) {
        
        std::unordered_map<TreeNode*, std::vector<TreeNode*>> adjList;

        std::unordered_set<TreeNode*> leafNodes;
        buildAdjList(root, adjList);

        for(const auto& [node, adjNodes] : adjList)
        {
            if(adjNodes.size() == 1 && node != root) leafNodes.emplace(node);
        }

        int returnVal = bfs(root, adjList, leafNodes, distance);

        return returnVal;
    }
};
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