Separate All

Sudhir uncle purchased three types of cold drinks. One for children, one for young people and one for old people but in the party hall, all the people are mixed together so it is difficult to serve the drinks. Consider the party hall as an array of integers, Children as 0, Young people as 1, and old people as 2. Segregate them in a manner so that all 0’s come first then all 1’s and then all 2’s.