Why does using cin instead of printf give me different results

This is an online judge problem of which the link is https://pintia.cn/problem-sets/994805342720868352/problems/994805406352654336.
The compiler I choose is g++.
the input range:
a, b and c in [−263,263 ]
t <= 10

#include <bits/stdc++.h>

using namespace std;

int main()
{
    //freopen("in.txt", "r", stdin);
    
    int t, tcase = 1;
    scanf("%d", &t);
    
    while(t--)
    {
        long long a, b, c;

        //there is an error here when replacing scanf with cin
        //scanf("%lld %lld %lld", &a, &b, &c);
        cin >> a >> b >> c;

        long long res = a + b;
        bool flag;
        if(a < 0 && b < 0 && res >= 0)flag = false;
        else if(a > 0 && b > 0 && res < 0)flag = true;
        else if(a + b > c)flag = true;
        else flag = false;

        if(flag == true)
        {
            printf("Case #%d: true\n", tcase++);
        }
        else
        {
            printf("Case #%d: false\n", tcase++);
        }
    }
    return 0;
}

Sometimes I use scanf, the program runs correctly, but when replacing it with cin, I get the wrong answer. This is an online judge problem, so I am sorry that I can not give the test data and the output information. Please tell me what is the problem, thank you very much.