Max in O(1)

#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;
int main( ) {
    int d;

    cout<<"Enter nunber of elements: ";
    cin>>d;
    vector <int> myVector(d);
    int res=0;
    int mx=0;
    for(unsigned int i=0;i<d;i++){
    cin>>res;
    mx=max(mx,res );
    }
    cout<<endl;
    cout<<mx;
    return 0;
}