# What is Morris Traversal for a Tree and how to implement one?

Technology CommunityCategory: Binary TreeWhat is Morris Traversal for a Tree and how to implement one?
VietMX Staff asked 5 months ago

Morris (In-Order) traversal is a tree traversal algorithm that does not employ the use of recursion or a stack. In this traversal, links are created as successors and nodes are printed using these links. Finally, the changes are reverted back to restore the original tree.

Basically a tree is threaded by making all right child pointers that would normally be null point to the inorder successor of the node (if it exists), and all left child pointers that would normally be null point to the inorder predecessor of the node. Consider:

Algorithm:

• Initialize the root as the current node curr.
• While curr is not NULL, check if curr has a left child.
• If curr does not have a left child, print curr and update it to point to the node on the right of curr.
• Else, make curr the right child of the rightmost node in curr‘s left subtree.
• Update curr to this left node.
Complexity Analysis

The way of looking at time complexity is to find out how many times a tree node will be traversed. As it is constant (3 times for a binary tree). So we are looking at O(n).

Implementation
public class MorrisPreorderTraversal {  static class Node { //static class to represent node structure    int data; //to store data of each node    Node left, right; //to point to left and right child of the node respectively    Node(int d) { //constructor for initialisation of each values      this.data = d;      this.left = null;      this.right = null;    }  }  public static void printPreorder(Node root) {    if (root == null) { //if tree is empty      return;    }    while (root != null) {      if (root.left == null) { //if left child is empty        System.out.print(root.data + " "); //traverse current node and        root = root.right; //move to it's right      } else {        Node curr = root.left; //to store inorder predecessor        while (curr.right != null && curr.right != root) {          curr = curr.right;        }        if (curr.right == root) { //if right child of inorder predecessor points to root node itself          curr.right = null;          root = root.right;        } else { //otherwise          System.out.print(root.data + " "); //traverse current node          curr.right = root; //make right child of inorder predecessor point to this node          root = root.left;        }      }    }  }  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);    printPreorder(root);  }}/*  -:Sample Input Tree:-                   1                  / \                 2   3                / \ / \               4  5 6  7-:Sample Output:-1 2 4 5 3 6 7 -:Time Complexity:-O(n) where n is the number of nodes in the given tree-:Space Complexity:-O(1)    */