Inorder Traversal of a Binary Tree using Recursion in C++

This C++ program, using recursion, performs recursive Inorder traversal of a Given Binary Tree.

Here is the source code of the C++ program to perform Inorder Traversal. The C++ program is successfully compiled and run on a Linux system. The program output is also shown below.

```
/*
```

 * C++ Program to Perform Inorder Recursive Traversal of a Given Binary Tree

```
 */
```
```
# include <iostream>
```
```
# include <cstdlib>
```
```
using namespace std;
```
```
/*
```
```
 * Node Declaration
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```
 */
```
```
struct node
```
```
{
```
```
    int info;
```
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    struct node *left;
```
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    struct node *right;
```
```
}*root;
```
```
 
```
```
/*
```
```
 * Class Declaration
```
```
 */
```
```
class BST
```
```
{
```
```
    public:
```
```
        void insert(node *, node *);
```
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        void inorder(node *);
```
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        void display(node *, int);
```
```
        BST()
```
```
        {
```
```
            root = NULL;
```
```
        }
```
```
};
```
```
/*
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```
 * Main Contains Menu
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```
 */
```
```
int main()
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```
{
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    int choice, num;
```
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    BST bst;
```
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    node *temp;
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    while (1)
```
```
    {
```

        cout<<"-----------------"<<endl;

        cout<<"Operations on BST"<<endl;

        cout<<"-----------------"<<endl;

        cout<<"1.Insert Element "<<endl;

        cout<<"2.Inorder Traversal"<<endl;

```
        cout<<"3.Display"<<endl;
```
```
        cout<<"4.Quit"<<endl;
```
```
        cout<<"Enter your choice : ";
```
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        cin>>choice;
```
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        switch(choice)
```
```
        {
```
```
        case 1:
```
```
            temp = new node;
```

            cout<<"Enter the number to be inserted : ";

```
            cin>>temp->info;
```
```
            bst.insert(root, temp);
```
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            break;
```
```
        case 2:
```

            cout<<"Inorder Traversal of BST:"<<endl;

```
            bst.inorder(root);
```
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            cout<<endl;
```
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            break;
```
```
        case 3:
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            cout<<"Display BST:"<<endl;
```
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            bst.display(root,1);
```
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            cout<<endl;
```
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            break;
```
```
        case 4:
```
```
            exit(1);
```
```
        default:
```
```
            cout<<"Wrong choice"<<endl;
```
```
        }
```
```
    }
```
```
}
```
```
 
```
```
 
```
```
/*
```
```
 * Inserting Element into the Tree
```
```
 */
```

void BST::insert(node *tree, node *newnode)

```
{
```
```
    if (root == NULL)
```
```
    {
```
```
        root = new node;
```
```
        root->info = newnode->info;
```
```
        root->left = NULL;
```
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        root->right = NULL;
```

        cout<<"Root Node is Added"<<endl;

```
        return;
```
```
    }
```
```
    if (tree->info == newnode->info)
```
```
    {
```

        cout<<"Element already in the tree"<<endl;

```
        return;
```
```
    }
```
```
    if (tree->info > newnode->info)
```
```
    {
```
```
        if (tree->left != NULL)
```
```
        {
```

            insert(tree->left, newnode);

```
        }
```
```
        else
```
```
        {
```
```
            tree->left = newnode;
```
```
            (tree->left)->left = NULL;
```
```
            (tree->left)->right = NULL;
```

            cout<<"Node Added To Left"<<endl;

```
            return;
```
```
        }
```
```
    }
```
```
    else
```
```
    {
```
```
        if (tree->right != NULL)
```
```
        {
```

            insert(tree->right, newnode);

```
        }
```
```
        else
```
```
        {
```
```
            tree->right = newnode;
```
```
            (tree->right)->left = NULL;
```

            (tree->right)->right = NULL;

            cout<<"Node Added To Right"<<endl;

```
            return;
```
```
        }
```
```
    }
```
```
}
```
```
 
```
```
/*
```
```
 * In Order Traversal
```
```
 */
```
```
void BST::inorder(node *ptr)
```
```
{
```
```
    if (root == NULL)
```
```
    {
```
```
        cout<<"Tree is empty"<<endl;
```
```
        return;
```
```
    }
```
```
    if (ptr != NULL)
```
```
    {
```
```
        inorder(ptr->left);
```
```
        cout<<ptr->info<<"  ";
```
```
        inorder(ptr->right);
```
```
    }
```
```
}
```
```
 
```
```
 
```
```
/*
```
```
 * Display Tree Structure
```
```
 */
```
```
void BST::display(node *ptr, int level)
```
```
{
```
```
    int i;
```
```
    if (ptr != NULL)
```
```
    {
```
```
        display(ptr->right, level+1);
```
```
        cout<<endl;
```
```
        if (ptr == root)
```
```
            cout<<"Root->:  ";
```
```
        else
```
```
        {
```
```
            for (i = 0;i < level;i++)
```
```
                cout<<"       ";
```
```
	}
```
```
        cout<<ptr->info;
```
```
        display(ptr->left, level+1);
```
```
    }
```
```
}
```

$ g++ inorder.cpp
$ a.out
 
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 7
Root Node is Added
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 3
Display BST:
 
Root->:  7
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 3
Node Added To Left
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 3
Display BST:
 
Root->:  7
              3
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 2
Inorder Traversal of BST:
3  7
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 9
Node Added To Right
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 3
Display BST:
 
              9
Root->:  7
              3
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 2
Inorder Traversal of BST:
3  7  9
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 2
Node Added To Left
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 1
Node Added To Left
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 3
Display BST:
 
              9
Root->:  7
              3
                     2
                            1
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 2
Inorder Traversal of BST:
1  2  3  7  9
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 8
Node Added To Left
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 3
Display BST:
 
              9
                     8
Root->:  7
              3
                     2
                            1
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 2
Inorder Traversal of BST:
1  2  3  7  8  9
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 1
Enter the number to be inserted : 11
Node Added To Right
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 3
Display BST:
 
                     11
              9
                     8
Root->:  7
              3
                     2
                            1
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 2
Inorder Traversal of BST:
1  2  3  7  8  9  11
-----------------
Operations on BST
-----------------
1.Insert Element
2.Inorder Traversal
3.Display
4.Quit
Enter your choice : 4