<p>Given the <code>root</code> of a binary tree, flatten the tree into a &quot;linked list&quot;:</p>

<ul>
	<li>The &quot;linked list&quot; should use the same <code>TreeNode</code> class where the <code>right</code> child pointer points to the next node in the list and the <code>left</code> child pointer is always <code>null</code>.</li>
	<li>The &quot;linked list&quot; should be in the same order as a <a href="https://en.wikipedia.org/wiki/Tree_traversal#Pre-order,_NLR" target="_blank"><strong>pre-order</strong><strong> traversal</strong></a> of the binary tree.</li>
</ul>

<p>&nbsp;</p>
<p><strong class="example">Example 1:</strong></p>
<img alt="" src="https://assets.leetcode.com/uploads/2021/01/14/flaten.jpg" style="width: 500px; height: 226px;" />
<pre>
<strong>Input:</strong> root = [1,2,5,3,4,null,6]
<strong>Output:</strong> [1,null,2,null,3,null,4,null,5,null,6]
</pre>

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

<pre>
<strong>Input:</strong> root = []
<strong>Output:</strong> []
</pre>

<p><strong class="example">Example 3:</strong></p>

<pre>
<strong>Input:</strong> root = [0]
<strong>Output:</strong> [0]
</pre>

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

<ul>
	<li>The number of nodes in the tree is in the range <code>[0, 2000]</code>.</li>
	<li><code>-100 &lt;= Node.val &lt;= 100</code></li>
</ul>

<p>&nbsp;</p>
<strong>Follow up:</strong> Can you flatten the tree in-place (with <code>O(1)</code> extra space)?
class Solution {
public:
    void preorder(TreeNode* root,vector<TreeNode*>&ans){
        if(root==NULL) return;
        ans.push_back(root);
        preorder(root->left,ans);
        preorder(root->right,ans);
    }
    void flatten(TreeNode* root) {
        if(root==NULL) return;
        vector<TreeNode*> ans;
        preorder(root,ans);
        int n=ans.size();
        root->left=NULL;
        for(int i=1;i<n;i++){
            root->right=ans[i];
            root=root->right;
            root->left=NULL;
            if(i==n-1) root->right=NULL;
        }
        return;
    }
};
17500