# Well commented Java solution, provide functions for both making the linked list in right subtree and in left subtree

• ``````public class Solution {
public void flatten(TreeNode root) {
flattenfunction(root);
}

/*
This method flattens a binary tree and constructs it into a linked list in the right subtree
@param current root used for the flatten function
@return the tree node that is the last one in the right subtree
*/
private TreeNode flattenfunction(TreeNode root){
if (root == null){
return root;
}
if (root.left != null){
//since we have to construct the binary tree in the order of pre-order traversal and we know that the left most nodes in the tree are always the nodes that are visited firstly (as the pre-order traversal algorithm like), so we just switch the left subtree of every level in the left subtree of the original root into the right side and recurse this process.
//after this process, we have move every left most nodes into the right place of the linked list
//switch left and right subtree
TreeNode temp = root.right;
root.right = root.left;
root.left = temp;
}
if (root.right != null){
//While there are nodes in the right subtree(after switch), we'll recurse the process with the right child as the root parameter.
//While there is no left child on this node, we can return the last node safely because without left subtree there can't be any nodes that can be switched to right subtree which make the right subtree deeper
//If there still is left subtree, we switch it to the right subtree of the last node and recurse the process. At the same time, we let the left subtree of the root to be null
//Node last stores the last node in the right subtree
TreeNode last = flattenfunction(root.right);
if (root.left == null){
return last;
}

last.right = root.left;
root.left = null;

return flattenfunction(last.right);
}
//if both left and right are null, then current root should be the last one.
return root;
}

/*
this method returns a linked list that is constructed on the left subtree of every node.
*/
private TreeNode leftflatten(TreeNode root){
if (root == null){
return root;
}
if (root.left != null){
//we return the last node by recursion
TreeNode last = leftflatten(root.left);
if (root.right != null){
//if there still exists a right subtree, we make it to be the left child of the last node and make the right child of the to be null
last.left = root.right;
root.right = null;
return leftflatten(last.left);
}
else {
return last;
}
}
if (root.right != null ){
//while root.left == null and root.right != null, we just switch the right subtree to the left subtree and execute the process.
root.left = root.right;
root.right = null;
return leftflatten(root.left);
}

//while both left and right child of the root is null, we know it should be the leaf node(last node) and return it
return root;
}
``````

}

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