private void addNodeToStack(Stack<BinaryNode> stack, BinaryNode node) {
    if (node != null) {
      stack.add(node);
    }
  }
Iterative solution for this algorithm. Using a tree traversal based on two parallel data
structures implemented using one Stack. The complexity order of this algorithm is O(N) in time
terms, as the previous one. But in space terms, the complexity order of this algorithm is O(N)
where N is the number of elements in the smallest tree because we are going to store this
elements in the stack.
src/main/java/com/github/pedrovgs/problem17/BinaryTreeEquals.java