求和1-100，但是某公司面试题

【叠甲】是某家公司的面试题，还挺出名的，题目简单，但搭配上面试官也透露出浓浓的pua味道。 ---------------也算秋招时候的趣闻了。---------------

/**  
* @author Bruce  
*/  
public class Summation {  
    private Summation(){}  
    private static volatile Summation instance = null;  
    public static Summation getInstance(){  
        if (instance == null){  
        synchronized (Summation.class){  
            if (instance == null){  
                instance = new Summation();  
                }  
            }  
        }  
        return instance;  
    }  
  
    public static Integer count(int start, int end){  
        return (start + end) % 2 == 1 ? 
            (((end + start - 1) >> 1) * ((end - start - 1) + 1) + end) :
            (((end + start) >> 1) * (end - start) + 1);  
    }  
  
}