Train Problem

public boolean train(List<String> in ,List<String> out, int number){boolean ret = true;if(in == null || out == null){return false;}if(in.size() != number || out.size() != number){return false;}Stack<String> s = new Stack<String>();int outstart = 0;int instart = 0;while(outstart < out.size()){s.push(in.get(instart++));while(s.size() > 0 && s.get(s.size() - 1).equals(out.get(outstart))){s.pop();outstart++;}if(outstart == out.size()){return true;}if(instart == in.size()){return false;}}return ret;}