<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) {}
 * };
 */
struct myTreeNode {
    int val;
    myTreeNode *parent;
    myTreeNode *left;
    myTreeNode *right;
    myTreeNode() : val(0), left(nullptr), right(nullptr), parent(nullptr) {}
    myTreeNode(int x) : val(x), left(nullptr), right(nullptr), parent(nullptr) {}
    myTreeNode(int x, myTreeNode *left, myTreeNode *right) : val(x), left(left), right(right), parent(nullptr) {}
};
class Solution {
public:
    unordered_set<myTreeNode*> leafs;
    
    int countPairs(TreeNode* root, int distance) {
        auto myRoot = buildMyTree(root);
        if( leafs.size() <= 1) return 0;
        int ans = 0;
        for(auto&leaf :leafs)
        {
            queue<pair<myTreeNode*, int>> Q;
            unordered_set<myTreeNode*> vis;
            
            vis.insert(leaf);
            Q.push({leaf, 0});
            while( Q.size() > 0 )
            {
                auto cur = Q.front(); Q.pop();
                if( cur.second > distance) break;
                if ( cur.second > 0 && leafs.count(cur.first) )
                {
                    ans++;
                }
                
                if ( cur.first->left != nullptr && !vis.count(cur.first->left) )
                {
                    vis.insert(cur.first->left);
                    Q.push({cur.first->left, cur.second+1});
                }
                if ( cur.first->right != nullptr && !vis.count(cur.first->right) )
                {
                    vis.insert(cur.first->right);
                    Q.push({cur.first->right, cur.second+1});
                }
                if ( cur.first->parent != nullptr && !vis.count(cur.first->parent) )
                {
                    vis.insert(cur.first->parent);
                    Q.push({cur.first->parent, cur.second+1});
                }                
            }
        }
        return ans/2;
    }
    myTreeNode* buildMyTree(TreeNode* root, myTreeNode* parent = nullptr)
    {
        myTreeNode* myNode = new myTreeNode(root->val);
        myNode->parent = parent;
        if( root->left  ) myNode->left  = buildMyTree(root->left , myNode);
        if( root->right ) myNode->right = buildMyTree(root->right, myNode);
        if( root->left == nullptr && root->right == nullptr) leafs.insert(myNode);
        return myNode;
    }
};
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