1.Create a Stack for storing the tree's nodes.
2. Start nested loops to traverse through the entire tree and its subtrees.
3. Run the outer loop infinitely and traverse the inner loop till the root becomes null.
4. Within the inner loop, push a node onto the stack and change it to its left node.
5. Outside the inner loop, start popping the nodes from the Stack. Insert the data of the node in an ArrayList that needs to be returned. Move to the right child.
6. If the Stack becomes empty, break out of the outer loop.
