Q:

Did Stephen King like Frank Darabont's ending of "The Mist"?

Frank Darabont's movie The Mist is based on a Stephen King short story The Mist. But the two do not have the same ending.
Here is how the short story ends (as per Wikipedia):

 David, Billy, Amanda, and elderly, yet tough, school teacher Hilda Reppler reach the car and leave Bridgton, driving south for hours through a mist-shrouded, monster-filled New England. After finding refuge for the night, David listens to a radio and, through the overwhelming static, possibly hears a single word broadcast: "Hartford". With that one shred of hope, he prepares to drive on into an uncertain future.

Whereas the movie has a much more definite ending (again as per Wikipedia):

 Driving through the mist, David finds his house destroyed and his wife dead. Devastated, he drives the group south, passing destroyed vehicles and seeing a gigantic six-legged, tentacled beast. When they run out of gas, the group decides there is no point in going on. David shoots the others rather than have them endure horrifying deaths, but is left with no bullet to use on himself. 

 He leaves the car and waits to be killed, but the mist suddenly recedes, revealing that the U.S. Army has arrived, rescued whatever survivors, and restored order. Among the survivors is the woman who left the store at the phenomenon's onset, accompanied by her two children. David breaks down with the realization that they were only moments from being rescued and had likely been driving away from help the entire time.

Did King approve of this ending?

A:

In a recent interview with Frank Darabont by Nick Schager for Yahoo Movies, Darabont first explains his reason for changing the ending:

When I first read Steve’s [Stephen King] story back a zillion years
  ago [in 1980’s Dark Forces anthology], I thought, “Wow, that’s a great
  story,” but I thought that for a movie, it should have a more
  conclusive sort of feeling. 

He points out that the story itself was the inspiration for it:

So I was trying to puzzle through what
  that conclusive ending would be, and he kind of lays the groundwork
  for that, actually. There’s a line of the story where he contemplates
  that eventuality. And I thought, well, that seems like a clear marker
  for me, that Steve laid in there.

The ending of the movie of course fits into a certain tradition:

When that came to me, it just felt like the kind of Twilight Zone
  ending that really stays with you. You know, “Time Enough at Last”
  where Burgess Meredith breaks his glasses — that kind of ending, where
  you’re like “Oh no, if he’d only waited two more minutes!” I liked the
  horrendous irony of it. 

And it was also fueled by Darabont's own feelings:

At that time, I was feeling a little bit
  pissed off at the world. There’s definitely a political element to
  that movie, which you don’t have to look too hard to see. Though it’s
  not a political movie, it’s in many ways a very political movie. I was
  feeling a little angry at the world, and at our country at that time
  [The Mist was released in 2007], so it felt like a valid way to end a
  movie. It doesn’t always have to be a happy ending. It shouldn’t
  always be a happy ending. Having grown up in the ’70s, it wasn’t
  always a happy ending. And I always loved endings like that.

Getting to the answer to the question: Darabont did ask King for input before filming:

But I thought, “OK, I’m going to let Steve decide. If Stephen King
  reads my script and says, ‘Dude, what are you doing, are you out of
  your mind? You can’t end my story this way,’ then I would actually not
  have made the movie.” But he read it and said, “Oh, I love this
  ending. I wish I’d thought of it.” He said that, once a generation, a
  movie should come along that just really pisses the audience off, and
  flips their expectations of a happy ending right on the head. He
  pointed to the original Night of the Living Dead as one of those
  endings that just scarred you.

