1. If root is null, return null.
2. Else, if the left node of the root is not null, find the size of the left subtree. If it's equal to k - 1, return root.
3. Else, if k is less than size, recurse through the left subtree. i.e. return findKthSmallest(root.left, k)
4. Recurse through the right subtree, with a new k -> k-leftSize-1 i.e.return findKthSmallest(root.right, k-leftSize-1)
