Binary Tree Traversals in Data Structures

In this section we will see different traversal algorithms to traverse keys present in binary search tree. These traversals are Inorder traversal, Preorder traversal, Postorder traversal and the level order traversal.
Suppose we have one tree like this −
The Inorder traversal sequence will be like − 5 8 10 15 16 20 23
The Preorder traversal sequence will be like − 10 5 8 16 15 20 23
The Postorder traversal sequence will be like − 8 5 15 23 20 16 10
The Level-order traversal sequence will be like − 10, 5, 16, 8, 15, 20, 23
inorderTraverse(root):
Begin
   if root is not empty, then
      inorderTraversal(left of root)
      print the value of root
      inorderTraversal(right of root)
   end if
End
preorderTraverse(root):
Begin
   if root is not empty, then
      print the value of root
      preorderTraversal(left of root)
      preorderTraversal(right of root)
   end if
End
postorderTraverse(root):
Begin
   if root is not empty, then
      postorderTraversal(left of root)
      postorderTraversal(right of root)
      print the value of root
   end if
End
levelOrderTraverse(root):
Begin
   define queue que to store nodes
   insert root into the que.
   while que is not empty, do
      item := item present at front position of queue
      print the value of item
      if left of the item is not null, then
         insert left of item into que
      end if
      if right of the item is not null, then
         insert right of item into que
      end if
      delete front element from que
   done
End
 Live Demo
#include<iostream>
#include<queue>
using namespace std;
class node{
   public:
      int h_left, h_right, bf, value;
      node *left, *right;
};
class tree{
   private:
      node *get_node(int key);
   public:
      node *root;
      tree(){
         root = NULL; //set root as NULL at the beginning
      }
      void inorder_traversal(node *r);
      void preorder_traversal(node *r);
      void postorder_traversal(node *r);
      void levelorder_traversal(node *r);
      node *insert_node(node *root, int key);
};
node *tree::get_node(int key){
   node *new_node;
   new_node = new node; //create a new node dynamically
   new_node->h_left = 0; new_node->h_right = 0;
   new_node->bf = 0;
   new_node->value = key; //store the value from given key
   new_node->left = NULL; new_node->right = NULL;
   return new_node;
}
void tree::inorder_traversal(node *r){
   if(r != NULL){ //When root is present, visit left - root - right
      inorder_traversal(r->left);
      cout << r->value << " ";
      inorder_traversal(r->right);
   }
}
void tree::preorder_traversal(node *r){
   if(r != NULL){ //When root is present, visit left - root - right
      cout << r->value << " ";
      preorder_traversal(r->left);
      preorder_traversal(r->right);
   }
}
void tree::postorder_traversal(node *r){
   if(r != NULL){ //When root is present, visit left - root - right
      postorder_traversal(r->left);
      postorder_traversal(r->right);
      cout << r->value << " ";
   }
}
void tree::levelorder_traversal(node *root){
   queue <node*> que;
   node *item;
   que.push(root); //insert the root at first
   while(!que.empty()){
      item = que.front(); //get the element from the front end
      cout << item->value << " ";
      if(item->left != NULL) //When left child is present, insert into queue
         que.push(item->left);
      if(item->right != NULL) //When right child is present, insert into queue
         que.push(item->right);
      que.pop(); //remove the item from queue
   }
}
node *tree::insert_node(node *root, int key){
   if(root == NULL){
      return (get_node(key)); //when tree is empty, create a node as root
   }
   if(key < root->value){ //when key is smaller than root value, go to the left
      root->left = insert_node(root->left, key);
   }else if(key > root->value){ //when key is greater than root value, go to the right
      root->right = insert_node(root->right, key);
   }
   return root; //when key is already present, do not insert it again
}
main(){
   node *root;
   tree my_tree;
   //Insert some keys into the tree.
   my_tree.root = my_tree.insert_node(my_tree.root, 10);
   my_tree.root = my_tree.insert_node(my_tree.root, 5);
   my_tree.root = my_tree.insert_node(my_tree.root, 16);
   my_tree.root = my_tree.insert_node(my_tree.root, 20);
   my_tree.root = my_tree.insert_node(my_tree.root, 15);
   my_tree.root = my_tree.insert_node(my_tree.root, 8);
   my_tree.root = my_tree.insert_node(my_tree.root, 23);
   cout << "In-Order Traversal: ";
   my_tree.inorder_traversal(my_tree.root);
   cout << "\nPre-Order Traversal: ";
   my_tree.preorder_traversal(my_tree.root);
   cout << "\nPost-Order Traversal: ";
   my_tree.postorder_traversal(my_tree.root);
   cout << "\nLevel-Order Traversal: ";
   my_tree.levelorder_traversal(my_tree.root);
}
In-Order Traversal: 5 8 10 15 16 20 23
Pre-Order Traversal: 10 5 8 16 15 20 23
Post-Order Traversal: 8 5 15 23 20 16 10
Level-Order Traversal: 10 5 16 8 15 20 23