<p>Given the <code>root</code> of a binary tree, the value of a target node <code>target</code>, and an integer <code>k</code>, return <em>an array of the values of all nodes that have a distance </em><code>k</code><em> from the target node.</em></p>

<p>You can return the answer in <strong>any order</strong>.</p>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://s3-lc-upload.s3.amazonaws.com/uploads/2018/06/28/sketch0.png" style="width: 500px; height: 429px;" />
<pre>
<strong>Input:</strong> root = [3,5,1,6,2,0,8,null,null,7,4], target = 5, k = 2
<strong>Output:</strong> [7,4,1]
Explanation: The nodes that are a distance 2 from the target node (with value 5) have values 7, 4, and 1.
</pre>

<p><strong class="example">Example 2:</strong></p>

<pre>
<strong>Input:</strong> root = [1], target = 1, k = 3
<strong>Output:</strong> []
</pre>

<p>&nbsp;</p>
<p><strong>Constraints:</strong></p>

<ul>
	<li>The number of nodes in the tree is in the range <code>[1, 500]</code>.</li>
	<li><code>0 &lt;= Node.val &lt;= 500</code></li>
	<li>All the values <code>Node.val</code> are <strong>unique</strong>.</li>
	<li><code>target</code> is the value of one of the nodes in the tree.</li>
	<li><code>0 &lt;= k &lt;= 1000</code></li>
</ul>

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:

    void populate(TreeNode* root, std::unordered_map<int, std::vector<int>>& graph){
        if(root->left != NULL) {
            graph[root->val].push_back(root->left->val);
            graph[root->left->val].push_back(root->val);
            populate(root->left, graph);
        }
        if(root->right != NULL) {
            graph[root->val].push_back(root->right->val);
            graph[root->right->val].push_back(root->val);
            populate(root->right, graph);
        }

    }
    vector<int> distanceK(TreeNode* root, TreeNode* target, int k) {
        std::unordered_map<int,std::vector<int>> graph;
        std::vector<int> ans;
        populate(root, graph);
        std::unordered_set<int> seen;
        std::deque<std::pair<int,int>> q;
        q.push_back(std::make_pair(target->val,0));
        seen.insert(target->val);
        while(!q.empty()){
            auto pair = q[0];
            q.pop_front();
            int node = pair.first;
            int dist = pair.second;
            if(dist>k) {
                break;
            }
            if(dist == k){
                ans.push_back(node);
            }
            for(auto child : graph[node]){
                if(!seen.contains(child)){
                    seen.insert(child);
                    q.push_back(std::make_pair(child,dist+1));
                }
            }
        }
        return ans;
    }
};
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