<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:
    int countPairs(TreeNode* root, int distance) {
        unordered_map<TreeNode*, vector<TreeNode*>> graph;
        unordered_set<TreeNode*> leafNodes;
        traverse(root, graph, leafNodes);

        int ans = 0;

        for(auto leaf: leafNodes)
        {
            unordered_set<TreeNode*> seen;
            queue<pair<TreeNode*, int>> q;
            q.push({leaf, 0});
            seen.insert(leaf);

            while(!q.empty())
            {
                auto it = q.front();
                q.pop();

                auto node = it.first;
                int level = it.second;

                if(level > distance) break;

                for(auto k: graph[node])
                {
                    if(leafNodes.find(k) != leafNodes.end() &&
                    seen.find(k) == seen.end() && 
                    leafNodes.find(node) == leafNodes.end() &&
                    level+1 <= distance)
                    {
                        cout<< node->val << " "<< k->val<<endl;
                        ans++;
                    }
                    else if(seen.find(k) == seen.end()) 
                    {
                        q.push({k, level+1});
                        seen.insert(k);
                    }
                }
            }
        }

        return ans/2;
    }

    void traverse(TreeNode* node, 
    unordered_map<TreeNode*, vector<TreeNode*>>& graph,
    unordered_set<TreeNode*>& leafNodes)
    {
        if(node == nullptr) return;

        if (node->left == nullptr && node->right == nullptr)
        {
            leafNodes.insert(node);
        }

        if (node->left != nullptr) 
        {
            graph[node].push_back(node->left);
            graph[node->left].push_back(node);

            traverse(node->left, graph, leafNodes);
        }
        if (node->right != nullptr) 
        {
            graph[node].push_back(node->right);
            graph[node->right].push_back(node);

            traverse(node->right, graph, leafNodes);
        }
    }
};
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