Working with Graphs & Trees

function topologicalSort(graph) {
  const visited = new Set();
  const temp = new Set();
  const order = [];
  
  function visit(vertex) {
    if (temp.has(vertex)) return false; // cycle
    if (visited.has(vertex)) return true;
    
    temp.add(vertex);
    
    for (let neighbor of graph[vertex]) {
      if (!visit(neighbor)) return false;
    }
    
    temp.delete(vertex);
    visited.add(vertex);
    order.unshift(vertex);
    return true;
  }
  
  for (let vertex in graph) {
    if (!visited.has(vertex) && !visit(vertex)) {
      return null; // graph has cycles
    }
  }
  
  return order;
}