In Salinger's The Catcher in the Rye, was the ending expected due to the course of the story? In what way does the ending shine light on the ideas presented in the novel? What is its contribution to understanding the meaning of the work?

At the end of The Catcher in the Rye, Holden is hospitalized for mental health reasons. He says:


"A lot of people, especially this one psychoanalyst guy they have here, keeps asking me if I'm going apply myself when I go back to school next September. It's such a stupid question, in my opinion."

It's clear that even though he's getting therapeutic help, he hasn't changed very much. 
The ending in many ways isn't surprising because at the very beginning of the book, Holden says, "I'll just tell you about this madman stuff that happened to me around last Christmas just before I got pretty run-down and had to come out here and take it easy." He is referring in an oblique way to his need for hospitalization.
In addition, throughout the book, there are many signs that Holden is alienated from everyone in his life and crying out for help. Even at the beginning of the book, he says about his brother, D.B., "Now he's out in Hollywood, D.B., being a prostitute." Holden is alienated from his parents, his brother, and then his friends at school when he's kicked out of Pencey Prep. After he leaves school, he has a series of disappointing encounters with women. He is also distraught after his brother, Allie, dies, and he says, "I was only thirteen, and they were going to have me psychoanalyzed and all, because I broke all the windows in the garage." On the night Allie died, Holden broke the windows in the garage, so it's apparent he has understandably had a really hard time with his brother's death. Afterward, he has no one to turn to and feels alienated. 
The ending implies that everything Holden has attempted to do to make himself feel better, including running around New York, dating girls, seeing his sister, and visiting the carousel, have not helped him heal. The contribution of the ending to the work is that Holden's professed disdain for many things and people in his life and his jaded attitude are not sustainable. He eventually has to care and connect with people in his life to heal himself.