#include <iostream>
using namespace std;
int main()
{
    int n;
    cin >> n;
    for (int i = 1; i <= n; i++)
    {
        for (int j = 1; j <= n - i; j++)
        {
            cout << "*";
        }
        for (int j = 1; j <= i * 2 - 1; j++)
        {
            cout << "@";
        }
        for (int j = 1; j <= n - i; j++)
        {
            cout << "*";
        }
        cout << endl;
    }
    return 0;
}

每层分别按顺序输出*，@和*

#include<bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    cin>>n;
    for (int i=n-1;i>=0;i--)
    {
        for (int j=1;j<=i;j++)
            cout<<"*";
        for (int j=1;j<=(n-i)*2-1;j++)
            cout<<"@";
        for (int j=1;j<=i;j++)
            cout<<"*";
        cout<<'\n';
    }
    return 0;
}