Given the task definition and input, reply with output. In this task, you need to reverse all words of a given length in the sentence. The number of letters in a word determine its length. For example, the length of the word "apple" is 5.

Sentence: 'a group of people on a train platform taking pictures of a departing train'. Reverse all words of length '1' in the given sentence.

a group of people on a train platform taking pictures of a departing train