import React from "react";import {BrowserRouter as Router, Switch, Route, Link, useParams, useRouteMatch}
    from "react-router-dom";export default function NestingExample() {
  return (
    <Router>
      <div>
        <ul><li><Link to="/">Home</Link></li>
               <li><Link to="/topics">Topics</Link></li></ul><hr />
        <Switch>
          <Route exact path="/"><Home /></Route>
          <Route path="/topics"><Topics /></Route>
        </Switch>
      </div>
    </Router>
  );}function Home() { return (<div><h2>Home</h2></div>);}function Topics() {  // The `path` lets us build <Route> paths that are relative to the parent route, while the    // `url` lets us build relative links.
  let { path, url } = useRouteMatch();
  return (
    <div>
      <h2>Topics</h2>
      <ul>
        <li><Link to={`${url}/rendering`}>Rendering with React</Link></li>
        <li><Link to={`${url}/components`}>Components</Link></li>
        <li><Link to={`${url}/props-v-state`}>Props v. State</Link></li>
      </ul>
      <Switch>
        <Route exact path={path}><h3>Please select a topic.</h3></Route>
        <Route path={`${path}/:topicId`}><Topic /></Route>
      </Switch>
    </div>
  );}function Topic() {  // The <Route> that rendered this component has a path of `/topics/:topicId`. The   // `:topicId` portion of the URL indicates a placeholder that we can/ get from   // `useParams()`.
  let { topicId } = useParams();
  return (<div><h3>{topicId}</h3></div>);}