feat: add Morris Inorder Traversal algorithm in JavaScript issue #1806 #1816

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	// Morris Inorder Traversal
	// Reference: https://www.geeksforgeeks.org/dsa/inorder-tree-traversal-without-recursion-and-without-stack/

	/*
	* Author: Saad Arqam
	* Morris Inorder Traversal Algorithm implementation in JavaScript
	*
	* Morris Traversal is a tree traversal algorithm that allows
	* inorder traversal without using recursion or a stack.
	* It achieves O(1) extra space by temporarily modifying
	* the tree structure (creating and removing "threads").
	*
	* Reference:
	* https://en.wikipedia.org/wiki/Threaded_binary_tree#Morris_traversal
	*/

	// Node class
	export class Node {
	constructor(val) {
	this.val = val
	this.left = null
	this.right = null
	}
	}

	// Morris Inorder Traversal function
	export function morrisTraversal(node) {
	const result = []
	let curr = node

	while (curr !== null) {
	if (curr.left === null) {
	result.push(curr.val)
	curr = curr.right
	} else {
	let predecessor = curr.left
	while (predecessor.right !== null && predecessor.right !== curr) {
	predecessor = predecessor.right
	}
	if (predecessor.right === null) {
	predecessor.right = curr
	curr = curr.left
	} else {
	predecessor.right = null
	result.push(curr.val)
	curr = curr.right
	}
	}
	}

	return result
	}

	// Example Tree:
	// 7
	// / \
	// 5 8
	// / \
	// 3 6
	// \
	// 9
	//
	// Morris inorder traversal: [3, 5, 6, 9, 7, 8]

46 changes: 46 additions & 0 deletions Trees/MorrisTraversal.test.js

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	import { describe, it, expect } from 'vitest'
	import { Node, morrisTraversal } from './MorrisTraversal.js'

	describe('Morris Inorder Traversal', () => {
	it('should return inorder traversal for a binary tree', () => {
	// Tree:
	// 4
	// / \
	// 2 5
	// / \
	// 1 3
	const root = new Node(4)
	root.left = new Node(2)
	root.right = new Node(5)
	root.left.left = new Node(1)
	root.left.right = new Node(3)

	const result = morrisTraversal(root)
	expect(result).toEqual([1, 2, 3, 4, 5])
	})

	it('should return empty array for null input', () => {
	const result = morrisTraversal(null)
	expect(result).toEqual([])
	})

	it('should handle larger tree', () => {
	// Tree:
	// 7
	// / \
	// 5 8
	// / \
	// 3 6
	// \
	// 9
	const root = new Node(7)
	root.left = new Node(5)
	root.right = new Node(8)
	root.left.left = new Node(3)
	root.left.right = new Node(6)
	root.left.right.right = new Node(9)

	const result = morrisTraversal(root)
	expect(result).toEqual([3, 5, 6, 9, 7, 8])
	})
	})

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