Four months ago, a 37-year-old was living his life as a homeless on the streets of New York City. A 23-year-old engineer, who was a complete stranger, approached him with two choices - $100 in cash or two months of free coding lessons. The homeless man chose the later and soon became an app developer, launching his own app called "Trees for Cars" for iOS as well as Android.

"Trees for Cars" is an environmentally-conscious carpool organizer that aims to make easier for New York commuters to share rides. Users can select a place on a map to meet ride shares which on the other sides are sent as requests that can be accepted and ignored.

The app is useful but what makes it extraordinary is the inspiring story of the developer who built it.

This is the story of Leo Grand, who was forced out of his apartment after losing his job at MetLife, and an enthusiastic programmer Patrick McConlogue, who taught the man coding lessons from scratch.

McConlogue passed by Grand sleeping on streets everyday on his way to work. One day he stopped by Grand to offer some help. He gave the homeless Grand the two choices and once he opted to learn how to code, MConlogue provided him with a used Chromebook laptop and a few text books. They started their lessons on the city benches near Chelsea Piers. Everyday McConlogue taught Grand for an hour and then headed off to work. Grand practiced what he learnt until his laptop battery drained.

The routine continued for around three-and-half months and finally writing 3,621 lines of code the duo were able to release their first mobile app.

"This is going to change my life in a magnificent way" said Grand after the app launch.

"Trees for Cars is a great way to build relationships, strengthen communities, help each-other financially and energy wise, all under the umbrella of saving the environment," Grand said in a statement.

The app is available on Apple App Store and Google Play store.

Meanwhile, Patrick's volunteer program has reached around 150 participants across the world who spend an hour each day for two months in teaching others to code, but he still hasn't figured out a way to keep up with the demand.