Vector not defined

I have added bits/stdc+.h and vector both. Still this error is coming . Can anyone tell me why this is happening.

#include <bits/stdc++.h>
#include<vector>
void rotate(int arr[], int n);

int main()
{
    int t;
    scanf("%d",&t);
    while(t--)
    {
        int n;
        scanf("%d",&n);
        int a[n] , i;
        for(i=0;i<n;i++)
        scanf("%d",&a[i]);
        rotate(a, n);
        for (i = 0; i < n; i++)
            printf("%d ", a[i]);
        printf("\n");
    }
        return 0;
}
// } Driver Code Ends


//User function Template for C++

void rotate(int arr[], int n)
{
    vector<int> a;
    a[0] = arr[n-1];
    for(int i = 0 ; i<n-1 ;i++)
      {
          a.insert(a.back(), arr[i]);
      }
      
   for(int j : a)
    cout<<j;
}

main.cpp:30:5: error: ‘vector’ was not declared in this scope vector a; ^~~~~~