c++ - What's wrong with my code about find the max one and the min one by vector

I wrote a code，it will find the max one and min one in an arr by vector
but I simply don't know what I've made an error.

Every time I try it return zero for me.lol

#include
#include
#include
using namespace std;
int main()
{
    int a[999];
    int n;

    cin >> n;
    for (int i = 1; i <= n; i++) {
        cin >> a[i]; 
    }
    vector arr(a[1], a[n]);
    int max = *max_element(arr.begin(), arr.end());
    int min = *min_element(arr.begin(), arr.end());
    int dis = max - min; cout << dis << endl;
    return 0;
}