Copyright (c) 1994 W. Brainerd, C. Goldberg, and J. Adams. All rights reserved. This file may not be copied without permission of the authors.
One of the big disadvantages of using a linked list to sort numbers is
that the resulting program has poor expected running time.
In fact, for the program
the expected running time is proportional to n2,
where n is the number of numbers to be sorted.
A much more efficient sorting program can be constructed
if a slightly more complicated data structure, the binary tree, is used.
The resulting program,
has an expected running time proportional to n log2 n
instead of n2.
It is quite difficult to write nonrecursive programs to process trees, so we will think of trees as recursive structures right from the start. Using this approach, a binary tree of integers is either empty or is an integer, followed by two binary trees of integers, called the left subtree and right subtree.
A blank box in the lower part of a node is understood to represent a null pointer.
When the next number is read, it is compared with the first. If it is less than the first number, it is placed as a node in the left subtree; if it is greater than or equal to the first number, it is placed in the right subtree. In our example, 113 < 265, so a node containing 113 is created and the left subtree pointer of the node containing 265 is set to point to it, as shown in Figure 8-10.
The next number is 467, and it is placed in the right subtree of 265 because it is larger than 265. The result is shown in Figure 8-11.
The next number is 264, so it is placed in the left subtree of 265. To place it properly within the left subtree, it is compared with 113, the occupant of the top of the left subtree. Since 264 >= 113, it is placed in the right subtree of the one with 113 at the top to obtain the tree shown in Figure 8-12.
The next number 907 is larger than 265, so it is compared with 467 and put in the right subtree of the node containing 467, as shown in Figure 8-13.
The final number 265 is equal to the number in the root node.
An insertion position is therefore sought in the right subtree of the root.
Since 265 < 467, it is put to the left of 467, as shown in Figure 8-14.
Notice that the two nodes containing the number 265 are not even adjacent,
nor is the node containing the number 264 adjacent to either node
with key 265.
This doesn't matter.
When the tree is printed, they will come out in the right order.
8.3.2 Type Declarations for Trees
The declaration for the node of a tree is similar to the declaration
for the node of a linked list,
except that the node must contain two pointers,
one to the left subtree and one to the right subtree.
As with lists, we could have
be a derived data type, which implies it must be a structure
with one component, a pointer to the node of a tree.
But just to be different, let's not put the tree operations in a module.
Having made this decision, the extra syntax needed to select the
pointer component using the
symbol clutters up the program enough that we will simply
declare things that would be trees to be pointers to a node,
the root node of the tree.
Thus, the declarations needed are
type node integer :: value type (node), pointer :: left, right end type node type (node), pointer :: t
The subroutine that inserts a new number into the tree is a straightforward implementation of the following informal recipe: if the tree is empty, make the new entry the only node of the tree; if the tree is not empty and the number to be inserted is less than the number at the root, insert the number in the left subtree; otherwise, insert the number in the right subtree.
recursive subroutine insert (t, number) type (node), pointer :: t ! Really a tree. integer, intent (in) :: number ! If (sub)tree is empty, put number at root if (.not. associated (t)) then allocate (t) t % value = number nullify (t % left) nullify (t % right) ! otherwise, insert into correct subtree else if (number < t % value) then call insert (t % left, number) else call insert (t % right, number) end if end subroutine insert
The recipe for printing the nodes of the tree follows from the way the tree has been built. It is simply to print in order the values in the left subtree of the root, print the value at the root node, then print in order the values in the right subtree. This subroutine is shown in the following complete program that sorts a file of integers by reading them all in, constructing an ordered binary tree, and then printing out the values in the tree in order.
program tree_sort ! Sorts a file of integers by building a ! tree, sorted in infix order. ! This sort has expected behavior n log n, ! but worst case (input is sorted) n ** 2. implicit none type node integer :: value type (node), pointer :: left, right end type node type (node), pointer :: t ! A tree integer :: number, ios nullify (t) ! Start with empty tree do read (*, *, iostat = ios) number if (ios < 0) exit call insert (t, number) ! Put next number in tree end do ! Print nodes of tree in infix order call print_tree (t) contains recursive subroutine insert (t, number) type (node), pointer :: t ! A tree integer, intent (in) :: number ! If (sub)tree is empty, put number at root if (.not. associated (t)) then allocate (t) t % value = number nullify (t % left) nullify (t % right) ! Otherwise, insert into correct subtree else if (number < t % value) then call insert (t % left, number) else call insert (t % right, number) end if end subroutine insert recursive subroutine print_tree (t) ! Print tree in infix order type (node), pointer :: t ! A tree if (associated (t)) then call print_tree (t % left) print *, t % value call print_tree (t % right) end if end subroutine print_tree end program tree_sort
tree_sort, sorting different quantities of randomly generated integers to determine an approximate formula for the complexity of the program. It should be proportional to n log2 n.