LeetCode 1154. 一年中的第几天

class Solution {
public:
    int dayOfYear(string date) {
        int monthDays[12] = {31,28,31,30,31,30,31,31,30,31,30,31};
        int year = stoi(date.substr(0,4)), 
            month = stoi(date.substr(5,2)), 
            day = stoi(date.substr(8,2));
        int sum = 0;
        for(int i=1; i<month; i++)
            sum += monthDays[i-1];
        if(month > 2 && 
            ( (year % 4 == 0 && year % 100 != 0) || year % 400 == 0) )
            sum += 1;
        sum += day;
        return sum;
    }
};