（Easy) Day of the week - LeetCode

class Solution {
    public String dayOfTheWeek(int day, int month, int year) {
        
        
        //2019-9-8  Sunday.   
        
        // we need to get the date difference. In that way, we will be able to know what day that day is.
             int num_1 =    GetNum(day,month,year);
             int num_2 =    GetNum(8,9,2019);
             //System.out.println(num_2-num_1);
            int diff = Math.abs(num_2-num_1)%7;
           
           if(diff ==0){
               return "Sunday";
           }
        
        
        if(num_2 - num_1 >0){
            
             if(diff==1){
                
                return "Saturday";
            }
            else if(diff == 2){
                return "Friday";
            }
              else if(diff == 3){
                return "Thursday";
            }
              else if(diff == 4){
                return "Wednesday";
            }
              else if(diff == 5){
                return "Tuesday";
            }  else if(diff == 6){
                return "Monday";
            }
        }
        else{
            
            if(diff==1){
                
                return "Monday";
            }
            else if(diff == 2){
                return "Tuesday";
            }
              else if(diff == 3){
                return "Wednesday";
            }
              else if(diff == 4){
                return "Thursday";
            }
              else if(diff == 5){
                return "Friday";
            }  else if(diff == 6){
                return "Saturday";
            }
            
        }
        
        
            System.out.println(GetNum(3,3,1977));
        
            System.out.println(GetNum(8,9,2019));
        return "";
    }
    /*
                给定一个年份，判断是否是闰年。条件为：
        A：能被4整除，并且不能被100整除。或者
        B：能被400整除。*/
    
    static boolean Leap(int a){
        
        if(a%4==0&& a%100!=0){
            return true;
        }
        else if( a%400 ==0){
            return true;
        }
        else{
            return false;
        }
    }
    
    static int GetNum(int day, int month, int year){
            
           
        HashMap<Integer, Integer> map = new HashMap<Integer, Integer>();
        
        //Jan 31 days
        //Feb Leap year or not.  (28/29 days)
        //Mar 31 days
        //Apr 30 days
        //May 31 days
        //Jun 30 days
        //Jul 31 days
        //Aug 31 days
        //Sep 30 days
        //Oct 31 days
        //Nov 30 days
        //Dec 31 days
        
        map.put(1,31);
        map.put(3,31);
        map.put(4,30);
        map.put(5,31);
        map.put(6,30);
        map.put(7,31);
        map.put(8,31);
        map.put(9,30);
        map.put(10,31);
        map.put(11,30);
        map.put(12,31);
        
        
        if(Leap(year)){
            
            map.put(2,29);
        }
        else{
            map.put(2,28);
        }
        
        
          int sum = 0; 
            for(int i = 1977; i< year; i++){
                
                if(Leap(i)){
                     sum = sum + 366;
                }
                else{
                    sum = sum +365;
                }
               
            }
        
            for(int i = 1; i < month; i++){
                
                sum = sum + map.get(i);
                
            }
        
            sum = sum + day;
        
      return sum;
    }
}