I apologise in advance for a long post. I will include the essential part of multiple codes which I tried submitting for 785C once the contest got over. (In all my solutions I have defined int for long long beforehand so that's not an error) NOTE: If you haven't read the question, this is the link: QUESTION LINK

This is the code for the problem C which I submitted during the contest in GNU C++ 14 which got "Accepted".

25515250

int f(int x)
{
    int d=8*x+1;
    int x1=sqrt(d);
    x1=(x1-1)/2;
    if(x1*(x1+1)/2 == x)
        return x1;
    else
        return x1+1;
}
int main()
{
    int n,m;

    cin>>n>>m;
    if(n<=m)
    {
        cout<<n<<endl;
        return 0;
    }
    int x=n-m;
    cout<<m+f(x);

    return 0;
}

Once the contest got over looking at the number of solutions which failed System Tests, I tried submitting different variations in different compiler versions. This is the same solution which doesn't use the function call on GNU C++14 which gets a wrong answer on test 16.

25533640

int main()
{
    int n,m;

    cin>>n>>m;
    if(n<=m)
    {
        cout<<n<<endl;
        return 0;
    }
    int x=n-m,ans;
    int d=8*x+1;
    int x1=sqrt(d);
    x1=(x1-1)/2;
    if((x1*(x1+1))/2 ==x)
        ans= x1;
    else
         ans=x1+1;
    cout<<m+ans;

    return 0;
}

Now this is the solution in which I don't make any function call but gets "Accepted" on GNU C++.

25533909

int main()
{
    int n,m;

    cin>>n>>m;
    if(n<=m)
    {
        cout<<n<<endl;
        return 0;
    }
    int x=n-m,ans;
    int d=8*x+1;
    int x1=sqrt(d);
    x1=(x1-1)/2;
    if((x1*(x1+1))/2 >=x)
        ans= x1;
    else
         ans=x1+1;
    cout<<m+ans;

    return 0;
}

Then again, the following code works on the right logic but gets a Wrong Answer on test 16 for using double instead of long long on C++.

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int main()
{
    ll n, m;
    
    cin>>n>>m;
    
    if(n <= m)
    {
        cout<<n<<endl;
    }
    else
    {
        ll x = n - m;
        
        double y = sqrt(1+(8*x));
        y-=1.0;
        y/=2.0;
        ll yd = ceil(y);
        cout<<m+yd;
        
    }
    return 0;
}

The main problems are: 1. A code with a function call gets Accepted while the same logic without the function call and the instructions written inside main() function gets a wrong answer. 2. The same solution that I submit get different verdicts on GNU C++ and GNU C++14. 3. Using double in place of long long gives me wrong answer.

Thanks in advance to anyone who can help me with/explain these problems and sorry again for a long post.