private Set<String> generateOneCharFreSubseq(Collection<String> segments){
		
		Map<String, Integer> charAccessTimes = new HashMap<String, Integer>();
		Set<String> oneCharFreSubseqs = new HashSet<String>();
		
		// 统计每个字符出现的次数
		for(String segment: segments){
			for(int k = 0; k < segment.length(); k++){
				String currentChar = segment.substring(k, k + 1);
				
				// 统计每个字符出现的次数
				Integer count = charAccessTimes.get(currentChar) == null ? 0 : charAccessTimes.get(currentChar);
				charAccessTimes.put(currentChar, count + 1);
			}
		}
		
		// 过滤掉出现次数小于minSupport的子序列
		for(Map.Entry<String, Integer> entry: charAccessTimes.entrySet()){
			if(entry.getValue() >= minSupport){
				oneCharFreSubseqs.add(entry.getKey());
			}
		}
		
		return oneCharFreSubseqs;
	}