<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 {
  void getParent(TreeNode* root, int target, unordered_map<int, TreeNode*>& p, TreeNode* prev = NULL) {
    if (root == NULL) {
      return;
    }
    p[root->val] = prev;
    if (root->val == target) {
      return;
    }
    getParent(root->left, target, p, root);
    getParent(root->right, target, p, root);
  }

  void solve(TreeNode* root, int k, vector<int>& a) {
    if (root == NULL) {
      return;
    }

    if (k == 0) {
      a.push_back(root->val);
      return;
    }

    solve(root->left, k - 1, a);
    solve(root->right, k - 1, a);
  }
public:
    vector<int> distanceK(TreeNode* root, TreeNode* target, int k) {
      unordered_map<int, TreeNode*> p;
      getParent(root, target->val, p);

      vector<int> a;
      solve(target, k, a);
      while (p[target->val] != NULL) {
        TreeNode* _p = p[target->val];
        if (_p->left == target) {
          _p->left = NULL;
        } else if (_p->right == target) {
          _p->right = NULL;
        }
        solve(_p, --k, a);
        target = _p;
      }
      return a;
    }
};
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