4.9.2 Traversal by Iteration |

The recursive approach of the previous section is one valid way to
traverse a binary search tree in sorted order. This method has the
advantages of being simple and “obviously correct”. But it does have
problems with efficiency, because each call to *traverse_recursive*()
receives its own duplicate copies of arguments *action* and *param*, and
with convenience, because writing a new callback function for each
traversal is unpleasant. It has other problems, too, as already
discussed, but these are the ones to be addressed immediately.

Unfortunately, neither problem can be solved acceptably in C using a
recursive method, the first because the traversal function has to
somehow know the action function and the parameter to pass to it, and
the second because there is simply no way to jump out of and then back
into recursive calls in C.^{1} Our only option is to use an algorithm that does not
involve recursion.

The simplest way to eliminate recursion is by a literal conversion of the recursion to iteration. This is the topic of this section. Later, we will consider a slightly different, and in some ways superior, iterative solution.

Converting recursion into iteration is an interesting problem. There are two main ways to do it:

**tail recursion elimination**- If a recursive call is the last action taken in a function, then it is
equivalent to a
**goto**back to the beginning of the function, possibly after modifying argument values. (If the function has a return value then the recursive call must be a**return**statement returning the value received from the nested call.) This form of recursion is called tail recursion (see tail recursion). **save-and-restore recursion elimination**- In effect, a recursive function call saves a copy of argument values and
local variables, modifies the arguments, then executes a
**goto**to the beginning of the function. Accordingly, the return from the nested call is equivalent to restoring the saved arguments and local variables, then executing a**goto**back to the point where the call was made.

We can make use of both of these rules in converting
*traverse_recursive*() to iterative form. First, does
*traverse_recursive*() ever call itself as its last action? The answer
is yes, so we can convert that to an assignment plus a **goto** statement:

52. <Iterative traversal of BST, take 1 52> =staticvoidtraverse_iterative(structbst_node*node,bst_item_func*action,void*param)

{start:if(node!=NULL)

{traverse_iterative(node->bst_link[0],action,param);action(node->bst_data,param);node=node->bst_link[1];gotostart; } }

Sensible programmers are not fond of **goto**. Fortunately, it is easy to
eliminate by rephrasing in terms of a **while** loop:

53. <Iterative traversal of BST, take 2 53> =staticvoidtraverse_iterative(structbst_node*node,bst_item_func*action,void*param)

{while(node!=NULL)

{traverse_iterative(node->bst_link[0],action,param);action(node->bst_data,param);node=node->bst_link[1]; } }

This still leaves another recursive call, one that is not tail
recursive. This one must be eliminated by saving and restoring
values. A stack is ideal for this purpose. For now, we use a stack
of fixed size `BST_MAX_HEIGHT` and deal with stack overflow by
aborting. Later, we'll handle overflow more gracefully. Here's the
code:

54. <Iterative traversal of BST, take 3 54> =staticvoidtraverse_iterative(structbst_node*node,bst_item_func*action,void*param)

{structbst_node*stack[BST_MAX_HEIGHT];size_theight= 0;start:while(node!=NULL)

{if(height>=BST_MAX_HEIGHT)

{fprintf(stderr,"tree too deep\n");exit(EXIT_FAILURE); }stack[height++] =node;node=node->bst_link[0];gotostart;resume:action(node->bst_data,param);node=node->bst_link[1]; }if(height> 0)

{node=stack[--height];gotoresume; } }

This code, an ugly mash of statements, is a prime example of why **goto**
statements are discouraged, but its relationship with the earlier code
is clear. To make it acceptable for real use, we must rephrase it.
First, we can eliminate label *resume* by recognizing that it can only
be reached from the corresponding **goto** statement, then moving its code
appropriately:

55. <Iterative traversal of BST, take 4 55> =staticvoidtraverse_iterative(structbst_node*node,bst_item_func*action,void*param)

{structbst_node*stack[BST_MAX_HEIGHT];size_theight= 0;start:while(node!=NULL)

{if(height>=BST_MAX_HEIGHT)

{fprintf(stderr,"tree too deep\n");exit(EXIT_FAILURE); }stack[height++] =node;node=node->bst_link[0];gotostart; }if(height> 0)

{node=stack[--height];action(node->bst_data,param);node=node->bst_link[1];gotostart; } }

The first remaining **goto** statement can be eliminated without any other
change, because it is redundant; the second, by enclosing the whole
function body in an “infinite loop”:

56. <Iterative traversal of BST, take 5 56> =staticvoidtraverse_iterative(structbst_node*node,bst_item_func*action,void*param)

{structbst_node*stack[BST_MAX_HEIGHT];size_theight= 0;for(;;)

{while(node!=NULL)

{if(height>=BST_MAX_HEIGHT)

{fprintf(stderr,"tree too deep\n");exit(EXIT_FAILURE); }stack[height++] =node;node=node->bst_link[0]; }if(height== 0)break;node=stack[--height];action(node->bst_data,param);node=node->bst_link[1]; } }

This initial iterative version takes care of the efficiency problem.

**Exercises:**

**1.** Function *traverse_iterative*() relies on *stack*[], a stack of nodes
yet to be visited, which as allocated can hold up to `BST_MAX_HEIGHT`
nodes. Consider the following questions concerning *stack*[]:

- What is the maximum height this stack will attain in traversing a binary
search tree containing
*n*nodes, if the binary tree has minimum possible height? - What is the maximum height this stack can attain in traversing any
binary tree of
*n*nodes? The minimum height? - Under what circumstances is it acceptable to use a fixed-size stack as in the example code?
- Rewrite
*traverse_iterative*() to dynamically expand*stack*[] in case of overflow. - Does
*traverse_recursive*() also have potential for running out of “stack” or “memory”? If so, more or less than*traverse_iterative*() as modified by the previous part?

[1] This is possible in some other languages, such as Scheme, that support “coroutines” as well as subroutines.