核心代码就一行

#include <iostream>
using namespace std;
int main() {
    int h, m, s;
    char c;
    cin >> h >> m >> s >> c;
    (c == 'A') ? cout << h * 3600 + m * 60 + s : cout << 43200 + h * 3600 + m * 60 + s; //三目运算符
    return 0;
}