Preorder Traversal Without Recursion in C++

March 3, 2023

Preorder Traversal Without Recursion

There are three types of traversals in trees: Preorder,Inorder,Postorder. The traversals can be performed using recursion or stack. In this article, preorder traversal is performed using stack.

More About Preorder Traversal

Preorder traversal is a depth first algorithm.
We first print the value of the node,them move to the left subtree and finally to the right subtree.
If we need to explore more about the node itself before inspecting its leaves, we use preorder traversal.
Preorder traversal is used to find the prefix expression of an expression tree.

Algorithm:

If root is empty, return.
Push root to stack.
Continue until stack is not empty.
Print the top element of stack.
Add the right and the left child of the top element.

Inorder Postorder Preorder Traversal Examples

Preorder Tree Traversal in Binary Tree

Postorder Tree Traversal in Binary Tree

Inorder Tree Traversal without Recursion

Postorder Tree Traversal without Recursion

Code Implementation for Preorder Tree traversal without recursion

Run

#include<bits/stdc++.h>
using namespace std;
class Tree
{
public:
  int data;
  Tree *left = NULL, *right = NULL;
  // Constructor initialised
    Tree (int x)
  {
    data = x;
    left = NULL;
    right = NULL;
  }
};
void preorder (Tree * root)
{
  // If empty return;
  if (root == NULL)
    return;
  stack < Tree * >s;
  s.push (root);
  // Continue till stack is empty 
  while (!s.empty ())
    {
      Tree *temp = s.top ();
      // Print data
      cout << temp->data << " ";
      // Remove Data
      s.pop ();
      // Add right child
      if (temp->right != NULL)
	s.push (temp->right);
      // Add Left child
      if (temp->left != NULL)
	s.push (temp->left);
    }
}

int main ()
{
  Tree *root = new Tree (10);
  root->left = new Tree (20);
  root->right = new Tree (30);
  root->left->left = new Tree (40);
  root->left->right = new Tree (50);
  cout << "Preorder Traversal" << endl;
  preorder (root);
  return 0;
}

Output:

Preorder Traversal
10 20 40 50 30

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