Interestingly, from your description of a binary tree, it could also be defined in Haskell as:

data Tree = Node (Maybe Tree) Int (Maybe Tree)

Then a leaf node would look like:

node = Node Nothing 1 Nothing

For completeness, we should also mention that in OOP the second-closest thing to sum types is an inheritance hierarchy of a base type and derived types. The base type models the overall type and the derived types model the ‘either … or’ data cases. E.g. the above tree type and value in Java:

interface Tree {}
class Node extends Tree {
  private Tree _left; private int _value; private Tree _right;
  public Node(
    Optional<Tree> left, int value, Optional<Tree> right) {
    _left = left; _value = value; _right = right;
  }
  public Tree getLeft() { return _left; }
  public int getValue() { return _value; }
  public Tree getRight() { return _right; }
}
Tree node = new Node(Optional.empty(), 1, Optional.empty());

This syntactic difference really emphasises how much thought has gone into making sum types usable in ML-derived languages.