Day of the Week

<-E 1185> Day of the Week

class Solution {
public:
    string dayOfTheWeek(int day, int month, int year) {
        vector<int> mon{0,31,28,31,30,31,30,31,31,30,31,30,31};
        vector<string> dayofweek{"Sunday", "Monday", "Tuesday", 
                           "Wednesday", "Thursday", "Friday", "Saturday"};
        int sum=4;
        
        for(int i=1971;i<year;i++){
          if((((i % 4 == 0) && (i % 100 != 0)) ||
             (i % 400 == 0)))
                sum+=366;
            else
                sum+=365;
        }
        
        for(int i=1;i<month;i++){
            if(i==2 && (((year % 4 == 0) && (year % 100 != 0)) ||
             (year % 400 == 0)))
                sum++;
            sum+=mon[i];
        }
        
        sum+=day;
        return dayofweek[sum%7];
    }
};