seq.nth
seq.nth.cc
#include<algorithm>
int main() {
std::vector<int> xs { 1, 2, 2, 2, 3, 4, 5, 5 };
// 0 1 2 3 4 5 6 7
int n = 4;
auto pos = begin(xs) + n;
// std::nth_element(Iter, Iter, Iter);
// 自身を書き換えるので註意
std::nth_element(begin(xs), pos, end(xs));
std::cout << n << "-th element is " << *pos << std::endl;
// 4-th element is 3
}