先判断闰年，如果是就把2月天数变为29，最后累加求和即可

#include <bits/stdc++.h>
using namespace std;
int yue[13] = {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31};
int ans;
int a, b, c;
char c1, c2;
int main()
{
    cin >> a >> c1 >> b >> c2 >> c;
    if ((a % 4 == 0 && a % 100 != 0) || (a % 400 == 0))
        yue[2] = 29;
    for (int i = 1; i < b; i++)
        ans += yue[i];
    cout << ans + c;
    return 0;
}

可耻的打表（月份而已，虚惊亿场）诶，这是AC呀！

#include <bits/stdc++.h>
using namespace std;
int a[13] = {0,31,28,31,30,31,30,31,31,30,31,30,31};
int main()
{
    int y,m,d;
    char gang;
    cin >> y >> gang >> m >> gang >> d;
    int day = d;
    if ((y % 4 == 0 && y % 100 != 0) || (y % 400 == 0)) a[2] = 29;
    for (int i = 1;i < m;i++)
    {
        day += a[i];
    }
    cout << day;
}