使用React-Router-4，如何以编程方式设置索引路由`/`

import React from 'react';
import { BrowserRouter as Router, Route } from 'react-router-dom';

const App = () => (
  <Router>
    <div>
      <Route exact path="/" component={Home} />
      <Route path="/about" component={About} />
      <Route path="/contact" component={Contact} />
    </div>
  </Router>
);

const Home = () => (
  <div>
    <h2>Home</h2>
    {/* Home页面的内容 */}
  </div>
);

const About = () => (
  <div>
    <h2>About</h2>
    {/* About页面的内容 */}
  </div>
);

const Contact = () => (
  <div>
    <h2>Contact</h2>
    {/* Contact页面的内容 */}
  </div>
);

export default App;