print_tree_goto_state.cpp

#include<stdio.h>
#include<stdlib.h>

#include<utility>

struct tree {
  tree(int e, tree* l, tree *r) : element(e), left(l), right(r) {}
  
  int element;
  tree *left;
  tree *right;
};

enum DIR { LEFT_CHILD, RIGHT_CHILD };

struct stack {
  stack(DIR d, tree *v, stack *p) : dir(d), val(v), parent(p) {}
  
  DIR dir;
  tree *val;
  stack *parent;
};

std::pair<DIR, tree*> pop(stack*& s) {
  auto res = std::make_pair(s->dir, s->val);
  stack *tmp = s;
  s = s->parent;
  delete tmp;
  return res;
}

void push(DIR dir, tree *t, stack*& s) {
  s = new stack(dir, t, s);
}

void print_tree(tree *t) {
  tree *cur = t;
  stack *s = NULL;
  
begin_print_node:
  if (cur == NULL) {
    goto dispatch_stack_frame;
  } else {
    printf("(");
    push(LEFT_CHILD, cur, s);
    cur = cur->left;
    goto begin_print_node;
  }
  
print_element:
  printf(" %d ", cur->element);
  push(RIGHT_CHILD, cur, s);
  cur = cur->right;
  goto begin_print_node;
    
finish_print_node:
  printf(")");
  goto dispatch_stack_frame;

dispatch_stack_frame:
  std::pair<DIR, tree*> top_frame;
  if (s == NULL) goto end;
    
  top_frame = pop(s);
  cur = top_frame.second;
  if (top_frame.first == LEFT_CHILD) goto print_element;
  else                               goto finish_print_node;
  
  
end:
  return;
}


int main() {
  tree *a = new tree(1, NULL, NULL);
  tree *b = new tree(2, NULL, a);
  tree *c = new tree(3, a, b);

  print_tree(c);
  
  
  return 0;
}