Serve html file in nodejs server

I've been doing fine until I try to separate my code into routes, controllers and etc. Now I'm getting an error when I try to load the html file. When I go to the link http://localhost:3000/ I'm getting this error Error: ENOENT: no such file or directory, stat '/views/index.html'

This is my routes.js code

module.exports = function (app) {
    var userController = require('../controllers/userController');

    // app.use(require('express').static('../app/views/index.html'));

    app.get('/', userController.renderHomePage);

    app.post('/find', userController.getUser);
    app.get('/get', userController.getUsers);
    app.post('/add', userController.addUser);
}

And here's my userController.js file

var mongoose = require('mongoose');
var User = require('../models/user');

var express = require('express');

var app = express();
app.use(express.static('../app'));

exports.renderHomePage = function (req, res) {
    res.sendFile('/views/index.html');
}

exports.addUser = function(req,res){
    console.log(req.body);

    var newUser = new User({
        name : req.body.name,
        username : req.body.username,
        password : req.body.password
    });

    newUser.save(function(err){
        if(err){
            console.log(err);
        }
        else{
            console.log("User Saved successfully");
        }
    });

    res.send(req.body);
};

exports.getUsers = function (req, res) {
    // body...
    User.find({}, function(error, users){
        if(error){
            console.log(error);
        }
        else{
            res.send(users);
        }
    })
};

exports.getUser = function (req, res) {
    // body...
    console.log(req.body);
    var data = req.body.username;

    User.find({username : data}, function(err, user){
        if(err){
            throw err
        }
        else{
            console.log(user);
            res.send(user);
        }
    } );
};

Here's my server.js

var express = require('express');
// var mongoose = require('mongoose');
var bodyParser = require('body-parser');
var PORT = process.env.PORT || 3000;

var app = express();

var routes = require('./api/routes/routes');
routes(app);

var database = require('./config/database');

app.use(bodyParser.json());

app.listen(PORT, function(){
    console.log("Server is running on port "+PORT);
})

And here's my folder structure.

Server starting without an error. And I thought I've given the paths correctly. But I'm getting the error. Can anyone help me with this ? Thanks.

EDIT : This is how I've linked my script to the html file

<script src="/script/app.js"></script>