<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 dfsFindParent(TreeNode* ptr, unordered_map<TreeNode*, TreeNode*> &parent, unordered_set<TreeNode*> &leaf)
    {
        if(ptr == nullptr)
        return;

        if(!ptr->left && !ptr->right)
        leaf.insert(ptr);

        dfsFindParent(ptr->left, parent, leaf);
        dfsFindParent(ptr->right, parent, leaf);

        if(ptr->left)
        parent[ptr->left] = ptr;

        if(ptr->right)
        parent[ptr->right] = ptr;
    }

    int countPairs(TreeNode* root, int distance) 
    {
        unordered_map<TreeNode*, TreeNode*> parent;
        unordered_set<TreeNode*> leaf;

        dfsFindParent(root, parent, leaf);

        // for(auto p: parent)
        // cout << p.first->val << ": " << p.second->val << endl;
        // cout << endl;

        // for(auto node: leaf)
        // cout << node->val << " ";

        int c = 0;
        for(auto &i: leaf)
        {
            unordered_set<TreeNode*> vis;
            queue<pair<TreeNode*, int>> q;
            q.push({i, 0});
            vis.insert(i);

            while(!q.empty())
            {
                TreeNode* node = q.front().first;
                int dist = q.front().second;
                q.pop();

                if((dist <= distance) && (node != i) && (leaf.find(node) != leaf.end()))
                ++c;

                if(dist < distance)
                {
                    if(node->left && (vis.find(node->left) == vis.end()))
                    {
                        q.push({node->left, dist + 1});
                        vis.insert(node->left);
                    }

                    if(node->right && (vis.find(node->right) == vis.end()))
                    {
                        q.push({node->right, dist + 1});
                        vis.insert(node->right);
                    }

                    if((parent.find(node) != parent.end()) && (vis.find(parent[node]) == vis.end()))
                    {
                        q.push({parent[node], dist + 1});
                        vis.insert(parent[node]);
                    }
                }
            }
        }

        return c/2;
    }
};
96968