# Trees

In a `Tree`

, the running time for most operations is **O(logN)**

## 4.1 Preliminaries

- A
`tree`

can be defined in several ways. One natural way to define a tree is**recursively**. - A
`tree`

is a collection of*nodes*. - If not empty, a
`tree`

consists of a distinguished node*r*, called the**root**, and zero or more nonempty (sub)trees*T1*,*T2*, …. ,*Tk*, each of whose roots are connected by a directed*edge*from*r* - The
*root*of each subtree is said to be a**child**of*r*, and*r*is the parent of each subtree root.

**Figure 4.1** A tree

### 4.1.1 Implementation of Trees

The solution is simple: Keep the children of each node in a linked list of tree nodes.

```
#define ElementType int
typedef struct TreeNode* PtrToNode;
struct TreeNode
{
ElementType Element;
PtrToNode FirstChild;
PtrToNode NextSibling;
};
```

`FirstChild`

points to the head of its child
`NextSibling`

points to its brother in the linked list

### 4.1.2 Tree Traversals with an Application

The structure of a Unix file system.
`/usr`

**Codes to imply this idea:**

```
static void ListDir( DirectoryOrFile D, int Depth )
{
if (D is a legitimate entry)
{
PrintName(D, Depth);
if (D is an directory )
{
for each child, C of D
ListDir(C, Depth+1);
}
}
}
void ListDirectory( DirectoryOrFile D )
{
ListDir(D, 0);
}
static void SizeOfDirectory( DirectoryOrFile D )
{
int TotalSize;
TotalSize = 0;
if( D is a legitimate entry )
{
TotalSize = FileSize( D );
if( D is a directory )
for each child C of D
TotalSize += SizeOfDirectory( C );
}
return TotalSize;
}
```

## 4.2 Binary Trees

A binary tree is a tree in which no node can have more than two children.

### 4.2.1 Implementation

- Because a binary tree has most two children, we can keep direct pointers to them.
- In that a node is a structure consists of the
*Key*information plus two pointers (`left`

and`right`

) to other nodes.

```
typedef struct TreeNode* PtrToNode;
typedef struct PtrToNode Tree;
struct TreeNode
{
ElementType Element;
Tree Left;
Tree Right;
};
```

### 4.2.2 Expression Trees

We can produce an (overly parenthesized) infix expression by recursively producing a parenthesized left expression, then print out the operator at the root, and finally recursively producing a right parenthesized expression.

This general strategy is known as an *inorder* traversal; it is easy to remember because of the type of expression it produces.

There are two alternative strategies. See the book for more examples.

#### Constructing an Expression Tree

We now give an algorithm to convert a postfix expression into an expression tree.
As an example, suppose the input is:
`a b + c d e + * *`

Codes can be found here.

## 4.3 AVL Tree

An `AVL`

tree is a search tree with a *balance* condition.

- Require the left and right subtrees have the same height
- Every node must have left and right trees with the same height.

An `AVL`

tree is identical to a binary search tree, except that for every node in the tree, the height of the left and right subtrees can differ at most 1. (The height of a empty tree is defined to be `-1`

)
**Figure: A AVL tree**

**Note: Balanced factor** (Wikipedia)
In a binary tree the balance factor of a node N is defined to be the height difference

```
BalanceFactor(N) := Height(RightSubtree(N)) – Height(LeftSubtree(N))
```

of its two child subtrees. A binary tree is defined to be an AVL tree if the invariant

```
BalanceFactor(N) ∈ {–1,0,+1}
```

holds for every node N in the tree.

A node N with BalanceFactor(N) < 0 is called “left-heavy”, one with BalanceFactor(N) > 0 is called “right-heavy”, and one with BalanceFactor(N) = 0 is sometimes simply called “balanced”.

The minimum number of nodes, *S(h)* , in an `AVL`

tree of height **h** is given by

```
S(h) = S(h-1) + S(h-2) + 1
```

- Thus, all the tree operations can be performed in
**O()**time, except possibly insertion( we will assume lazy deletion ). When we do a insertion, we need to update all the balancing information for the nodes on the path back to the`root`

, but the reason that insertion is potentially difficult is that inserting a node could violate the`AVL`

Tree property. - If this is the case, then the property has to be resorted before the insertion step is considered over. It turns out that this can always be done with a simple modification to the tree, known as a
**rotation** - After insertion, only nodes that are on the path from the insertion point to the root might have their balance altered because only those nodes have their subtrees altered.
- As we follow the path up to the root and update the balancing information, we may find a node whose new balance violates the
`AVL`

condition.

Lets call the node that must be rebalanced . Since any node has at most two children, and a height imbalance requires that ’s two subtrees’ height differ by two, it is easy to see that a violation might occur in four cases:

- An insertion into the left subtree of the left child of ;
- An insertion into the right subtree of the left child of .
- An insertion into the left subtree of the right child of .
- An insertion into the left subtree of the right child of .

The first case (`left-left`

`right-right`

) is fixed by a *single rotation* of the tree and the second case (`left-right`

`right-left`

) is handled by a more complex *double rotation*.

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