Binary Tree – Preorder Traversal – Non Recursive Approach

Objective: Given a binary tree, write a non recursive or iterative algorithm for preorder traversal.

Tree Traversals - Preorder
Tree Traversals – Preorder

Example:

Earlier we have seen “What is preorder traversal and recursive algorithm for it“, In this article we will solve it with iterative/Non Recursive manner.

Since we are not using recursion, we will use the Stack to store the traversal, we need to remember that preorder traversal is, first traverse the root node then left node followed by the right node.

Pseudo Code:

  1. Create a Stack.
  2. Print the root and push it to Stack and go left i.e root=root.left and till it hits the NULL.
  3. If root is null and Stack is empty Then
    1. return, we are done.
  4. Else
    1. Pop the top Node from the Stack and set it as, root = popped_Node.
    2. Go right, root = root.right.
    3. Go to step 2.
  5. End If

See the animated image below and code for more understanding.

Preorder traversal
Preorder traversal

 

Complete Code:

import java.util.Stack;
public class PreOrderTree {
public void preOrderRecursive(Node root) {
if (root != null) {
System.out.print(root.data + " ");
preOrderRecursive(root.left);
preOrderRecursive(root.right);
}
}
public void preorderIteration(Node root) {
Stack<Node> s = new Stack<Node>();
while (true) {
// First print the root node and then add left node
while (root != null) {
System.out.print(root.data + " ");
s.push(root);
root = root.left;
}
// check if Stack is emtpy, if yes, exit from everywhere
if (s.isEmpty()) {
return;
}
// pop the element from the stack and go right to the tree
root = s.pop();
root = root.right;
}
}
public static void main(String[] args) {
Node root = new Node(1);
root.left = new Node(2);
root.right = new Node(3);
root.left.left = new Node(4);
root.left.right = new Node(5);
root.right.left = new Node(6);
root.right.right = new Node(7);
PreOrderTree i = new PreOrderTree();
i.preOrderRecursive(root);
System.out.println();
i.preorderIteration(root);
}
}
class Node {
int data;
Node left;
Node right;
public Node(int data) {
this.data = data;
}
}

view raw
PreOrderTree.java
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Output:

1 2 4 5 3 6 7
1 2 4 5 3 6 7