task1

const getTree = (flatArray) => {
    const map = new Map();
    const tree = [];

    for (const node of flatArray) {
        map.set(node.id, { ...node, child: [] });
    }

    for (const node of flatArray) {
        const { id, parentId } = node;
        if (parentId === null || parentId === undefined) {
            tree.push(map.get(id));
        } else {
            const parent = map.get(parentId);
            if (parent) {
                parent.child.push(map.get(id));
            }
        }
    }

    return tree;
};

const flatArray = [
    { id: 1, parentId: null },
    { id: 2, parentId: 1 },
    { id: 3, parentId: 1 },
    { id: 4, parentId: 2 },
    { id: 5, parentId: 2 },
    { id: 6, parentId: null },
    { id: 7, parentId: 6 },
];

const tree = getTree(flatArray);
console.log(JSON.stringify(tree, null, 2));