content: import React from 'react';
  import { Link, Route, Switch } from 'react-router-dom';
  
  class Topic extends React.PureComponent {
    render() {
      return (
        <div>
          <h3>ID: {this.props.match.params.id}</h3>
        </div>
      )
    }
  }
  
  class Topics extends React.PureComponent {
    render() {
      return (
        <div>
          <h2>主题</h2>
          <ul role="nav">
            <li><Link to="/topics/one">主题一</Link></li>
            <li><Link to="/topics/two">主题二</Link></li>
          </ul>
  
          <hr/>
  
          <Switch>
            <Route exact path='/topics'/>
            <Route path='/topics/:id' component={Topic}/>
          </Switch>
        </div>
      )
    }
  }
  export default Topics;
  