Flatten Binary Tree to Linked List | LeetCode | Java

if(root==null)
return;

flatten(root.right);
flatten(root.left);
root.right = prevNode;
root.left = null;
prevNode = root;
}
}

if(root==null)
return;

Stack<TreeNode> stack = new Stack<>();

stack.add(root);

while(!stack.isEmpty()){

TreeNode currentNode = stack.pop();

if(currentNode.right!=null)
stack.push(currentNode.right);

if(currentNode.left!=null)
stack.push(currentNode.left);

if(!stack.isEmpty())
currentNode.right = stack.peek();

currentNode.left = null;
}
}
}