Implement a function that performs a topological sort on a directed graph (in adjacency list format), where each key represents a node and its value is an array of nodes reached by outgoing edges from that node.

Return an array containing every node such that, for each directed edge from A to B, A appears before B. Return an empty array when the graph is empty or contains a cycle.

When multiple nodes have no remaining incoming edges, process them in first-in, first-out discovery order. Initially discover nodes in object-key order, and discover newly available neighbors in their adjacency-array order.

Examples

const graph1 = {
  A: ['B', 'C'],
  B: ['C', 'D', 'E'],
  C: ['F'],
  D: [],
  E: ['F'],
  F: [],
};
topologicalSort(graph1); // ['A', 'B', 'C', 'D', 'E', 'F']

const graph2 = {
  A: ['B', 'C'],
  B: ['C', 'D'],
  C: ['D'],
  D: ['E'],
  E: ['F'],
  F: [],
};
topologicalSort(graph2); // ['A', 'B', 'C', 'D', 'E', 'F']

const graph3 = {
  A: [],
  B: ['A'],
  C: ['B'],
  D: ['C'],
  E: ['D'],
  F: ['E'],
};
topologicalSort(graph3); // ['F', 'E', 'D', 'C', 'B', 'A']

A Queue data structure is also provided for you at the bottom of the skeleton code.