algo.ts

const forest = [/* ... */];

let forestKey = 0;
let childKey = 0;
let isChild = {};

// O(n) loop
while (true) {
    // O(1) : access by key to a hash map
    if (typeof forest[forestKey] === 'undefined') {
        break;
    }

    // O(1) + O(1) : access by key to a hash map
    if (typeof forest[forestKey].children[childKey] === 'undefined') {
        ++forestKey;
        childKey = 0;
        continue;
    }

    ++childKey;

    // O(1) : set by key of a hash map
    isChild[forest[forestKey].children[childKey]] = true;
}

let rootNodes = [];

// O(n) loop
for (let i in forest) {
    // O(1) : access by key to a hash map
    if (typeof isChild[forest[i]] === 'undefined') {
        rootNodes.push(forest[i]);
    }
}

return rootNodes;