How to find the k-th element of a subrange of an array/vector?

[Qbx]

Namely, given an array arr[1..n] of different integer numbers,
your program must answer a series of questions Q(i, j, k) in the form:
what would be the k-th number in arr[i..j] segment, if this segment was sorted?

Code:

#include <cstdio>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;


int main() {
    int arr[] = { 13, 9, 3, 8, 11, 42, 5, 6, 4, 19, 77, 1, 7 };

    vector<int> a(arr, arr + sizeof(arr) / sizeof(arr[0]));

return 0;
}

Any ideas to find it in the fastest way?

[charlie]

Sort, then use k (or k-1) as the index?