#include <string>
      #include <iostream>
      
      struct node
      { node * left;
        string data;
        node * right;
        node(node * L, string S, node * R): left(L), data(S), right(R) { } };
      
      void insert(node * & tree, string s)
      { if (tree==NULL)
          tree=new node(NULL, s, NULL);
        else if (s<tree->data)
          insert(tree->left, s);
        else
          insert(tree->right, s); }
      
      void recursive_print(node * tree)
      { // this is point 0
        if (tree==NULL) return;
        // this is point 1
        recursive_print(tree->left);
        // this is point 2
        cout << tree->data << "\n";
        // this is point 3
        recursive_print(tree->right);
        // this is point 4
      }
      
      struct SFprint
      { node * tree;
        int where;
        SFprint * prev;
        SFprint(node * T, int W, SFprint * P): tree(T), where(W), prev(P) { } };
      
      SFprint * create_enumeration(node * tree)
      { return new SFprint(tree, 0, NULL); }
      
      bool more_to_come(SFprint * & stack)
      { while (stack!=NULL)
        { node * tree = stack->tree;
          int where = stack->where;
          if (where==0)
          { if (tree==NULL)
            { stack = stack->prev; }
            else
            { stack->where = 1; } }
          else if (where==1)
          { stack->where = 2;
            stack = new SFprint(tree->left, 0, stack); }
          else if (where==2)
          { stack->where = 3;
            return true; }
          else if (where==3)
          { stack->where = 4;
            stack = new SFprint(tree->right, 0, stack); }
          else if (where==4)
          { stack=stack->prev; } }
        return false; }
      
      string get_current(SFprint * stack)
      { if (stack==NULL)
          return "";
        else
          return stack->tree->data; }
      
      string inputs1[] = { "freddy", "colin", "earnest", "zebidee", "kenny", "douglas", "brenda", "yolanda" };
      const int num_inputs1 = sizeof(inputs1)/sizeof(string);
      
      string inputs2[] = { "koala", "dog", "earwig", "yeti", "cat", "zebra", "bat", "frog" };
      const int num_inputs2 = sizeof(inputs2)/sizeof(string);
      
      void main(void)
      { node * root1 = NULL, * root2 = NULL;
        for (int i=0; i<num_inputs1; i+=1)
          insert(root1, inputs1[i]);
        for (int i=0; i<num_inputs2; i+=1)
          insert(root2, inputs2[i]);
        SFprint * enumer1 = create_enumeration(root1);
        SFprint * enumer2 = create_enumeration(root2);
        while (more_to_come(enumer1) && more_to_come(enumer2))
        { cout << get_current(enumer1) << " the " << get_current(enumer2) << "\n"; } }
      
$ CC nonrec.cpp
$ a.out
brenda the bat
colin the cat
douglas the dog
earnest the earwig
freddy the frog
kenny the koala
yolanda the yeti
zebidee the zebra