Better

#include<bits/stdc++.h>
int lowerBound(vector<int> arr, int n, int x) {
	int low=0;
    int high=n-1;
    int ans=n;

    while(low<=high){
        int mid=(low+high)/2;

        if(arr[mid]>=x){
            ans=mid;
            high=mid-1;
        }
        else{
            low=mid+1;
            }
        }
    return ans;
}
int upperBound(vector<int> &arr, int n, int x){
	int low=0;
    int high=n-1;
    int ans=n;

    while(low<=high){
        int mid=(low+high)/2;

        if(arr[mid]>x){
            ans=mid;
            high=mid-1;
        }
        else{
            low=mid+1;
            }
        }
    return ans;
}
pair<int, int> firstAndLastPosition(vector<int>& arr, int n, int k){
    int lb=lowerBound(arr, n, k);
    int ub=upperBound(arr, n, k);

    if(lb==n || arr[lb]!=k){
        return {-1,-1};
    }

    return {lb,ub-1};
}

Optimal

#include<bits/stdc++.h>
int lb(vector<int>& arr,int n,int k){
    int low=0;
    int high=n-1;
    int first=-1;
    while(low<=high){
        int mid=(low+high)/2;
        if(arr[mid]==k) {
            first=mid;
            high=mid-1;
        }
        else if(arr[mid]<k){
            low=mid+1;
        }
        else{
            high=mid-1;
        }
    }
    return first;
}
int ub(vector<int>& arr,int n,int k){
    int low=0;
    int high=n-1;
    int last=-1;
    while(low<=high){
        int mid=(low+high)/2;
        if(arr[mid]==k){
            last=mid;
            low=mid+1;
        }
        else if(arr[mid]<k){
            low=mid+1;
        }
        else{
            high=mid-1;
        }
    }
    return last;
}
pair<int, int> firstAndLastPosition(vector<int>& arr, int n, int k){
    if(lb(arr,n,k)==-1){
        return {-1,-1};
    }
    return {lb(arr,n,k) , ub(arr,n,k)};
}