tree.ts

class Thing {
    constructor(public id: number, public parentId: number) {
    }
}

/*
    1
   / \
  2   3
 /   / \
4   5   6
   /
  7
*/
const nodes: Thing[] = [
	new Thing(1, -1),
	new Thing(2, 1),
	new Thing(3, 1),
	new Thing(4, 2),
	new Thing(5, 3),
	new Thing(6, 3),
	new Thing(7, 5)	
];

class TreeNode<T> {
	constructor(public value: T, public children: TreeNode<T>[]) {
	}
	
	find(x: T): TreeNode<T> {
		if (this.value == x) {
			return this;
		}
		for (let i = 0; i < this.children.length; i++) {
			const found: TreeNode<T> = this.children[i].find(x);
			if (found) {
				return found;
			}
		}
		return null;
	}
}

function insert(tree: TreeNode<number>, thing: Thing): TreeNode<number> {
	const parentNode: TreeNode<number> = tree.find(thing.parentId);
	if (parentNode) {
		parentNode.children.push(new TreeNode(thing.id, []));
	}	
	return tree;
}

console.log(nodes.reduceRight(insert, new TreeNode(-1, [])));