<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:
    vector<int> distanceK(TreeNode* root, TreeNode* target, int k) {
        unordered_map<TreeNode*, TreeNode*> parentMap;
        vector<int> result;
        buildParentMap(root, nullptr, parentMap);
        unordered_set<TreeNode*> visited;
        queue<TreeNode*> q;
        q.push(target);
        visited.insert(target);
        int distance = 0;

        while (!q.empty()) {
            int size = q.size();
            if (distance == k) {
                for (int i = 0; i < size; i++) {
                    result.push_back(q.front()->val);
                    q.pop();
                }
                return result;
            }
            for (int i = 0; i < size; i++) {
                TreeNode* node = q.front();
                q.pop();
                if (node->left && visited.find(node->left) == visited.end()) {
                    visited.insert(node->left);
                    q.push(node->left);
                }
                if (node->right && visited.find(node->right) == visited.end()) {
                    visited.insert(node->right);
                    q.push(node->right);
                }
                if (parentMap[node] && visited.find(parentMap[node]) == visited.end()) {
                    visited.insert(parentMap[node]);
                    q.push(parentMap[node]);
                }
            }
            distance++;
        }
        return result;
    }
    
    void buildParentMap(TreeNode* node, TreeNode* parent, unordered_map<TreeNode*, TreeNode*>& parentMap) {
        if (!node) return;
        parentMap[node] = parent;
        buildParentMap(node->left, node, parentMap);
        buildParentMap(node->right, node, parentMap);
    }
};
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