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Flatten binary tree to linked list - GFG/README.md

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+# Flatten binary tree to linked list
+## Medium
+<div class="problems_problem_content__Xm_eO"><p><span style="font-size:18px">Given the root of a binary tree, flatten the tree into a "linked list":</span></p>
+
+<ul>
+	<li><span style="font-size:18px">The "linked list" should use the same Node class where the right child pointer points to the next&nbsp; &nbsp;node in the list and the left child pointer is always null.</span></li>
+	<li><span style="font-size:18px">The "linked list" should be in the same order as a pre-order traversal of the binary tree.</span></li>
+</ul>
+
+<p><strong><span style="font-size:18px">Example 1:</span></strong></p>
+
+<pre><span style="font-size:18px"><strong>Input : </strong>
+          1
+        /   \
+       2     5
+      / \     \
+     3   4     6</span>
+<span style="font-size:18px"><strong>Output :</strong>
+1 2 3 4 5 6 
+<strong>Explanation: </strong>
+After flattening, the tree looks 
+like this
+    1
+     \
+      2
+       \
+        3
+         \
+          4
+           \
+            5
+             \
+              6 
+Here, left of each node points 
+to NULL and right contains the 
+next node in preorder.The inorder 
+traversal of this flattened tree 
+is 1 2 3 4 5 6.</span></pre>
+
+<p><strong><span style="font-size:18px">Example 2:</span></strong></p>
+
+<pre><span style="font-size:18px"><strong>Input :</strong>
+        1
+       / \
+      3   4
+         /
+        2
+         \
+          5 
+<strong>Output :</strong> 
+1 3 4 2 5  
+<strong>Explanation : </strong>
+After flattening, the tree looks 
+like this 
+     1
+      \
+       3
+        \
+         4
+          \
+           2
+            \ 
+             5 
+Here, left of each node points 
+to NULL and right contains the 
+next node in preorder.The inorder 
+traversal of this flattened tree 
+is 1 3 4 2 5.</span></pre>
+
+<div><strong><span style="font-size:18px">Your task:</span></strong></div>
+
+<div><span style="font-size:18px">You don't have to read input or print anything. Your task is to complete the function flatten() which takes the root of the tree and flattens the tree into a linkes list without using any auxiliary space.</span></div>
+
+<div><span style="font-size:18px">Note : The driver code&nbsp;prints the inorder traversal of the flattened binary tree.</span></div>
+
+<div>&nbsp;</div>
+
+<div><span style="font-size:18px"><strong>Expected Time Complexity: </strong>O(n)</span></div>
+
+<div><span style="font-size:18px"><strong>Expected Auxiliary Space:</strong> O(1)</span></div>
+
+<div>&nbsp;</div>
+
+<div><strong><span style="font-size:18px">Constraints :</span></strong></div>
+
+<div><span style="font-size:18px">1&lt;=n&lt;=10^5</span></div>
+
+<div>&nbsp;</div>
+</div>