<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:
    unordered_map<TreeNode*, vector<TreeNode*>> graph;
    // unordered_set<int> leaves;
    vector<TreeNode*> leaves;

    void convertTreeToGraph(TreeNode* root)
    {
        if (!root) 
            return;

        if (root->left) {
            graph[root].push_back(root->left);
            graph[root->left].push_back(root);
        }

        if (root->right) {
            graph[root].push_back(root->right);
            graph[root->right].push_back(root);
        }

        if (!root->left && !root->right)
            // leaves.insert(root->val);
            leaves.push_back(root);

        convertTreeToGraph(root->left);
        convertTreeToGraph(root->right);
    }

    int countPairs(TreeNode* root, int distance) {
        convertTreeToGraph(root);

        queue<TreeNode*> nodes;
        unordered_set<TreeNode*> visited;
        int numPairs = 0, numCurrNodes, distFromLeaf;

        for (TreeNode* leaf : leaves)
        {
            // cout << leaf << " - ";

            // reset
            // numPairs = 0;
            numCurrNodes = 1;
            distFromLeaf = 0;
            visited.clear();
            while (!nodes.empty())
                nodes.pop();

            // init with start node
            nodes.push(leaf);
            visited.insert(leaf);

            // BFS
            while (!nodes.empty())
            {
                distFromLeaf++;
                if (distFromLeaf > distance)
                    break;

                while (numCurrNodes > 0)
                {
                    for (TreeNode* node : graph[nodes.front()])
                    {
                    
                        if (visited.find(node) == visited.end()) // not visited yet
                        {
                            nodes.push(node);
                            visited.insert(node);

                            // if (distFromLeaf <= distance && leaves.find(node) == leaves.end())
                            if (distFromLeaf <= distance && find(leaves.begin(), leaves.end(), node) != leaves.end())
                                numPairs++;
                        }
                    }

                    nodes.pop();

                    numCurrNodes--;
                }

                numCurrNodes = nodes.size();
            }

            cout << numPairs << endl;
        }

        return numPairs / 2;
    }
};
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