main.cpp

#include <iostream>
#include <algorithm>
#include <vector>
using namespace std;
int main() {
   vector<int> arr{1, 2, 3, 3, 5, 6, 6, 8, 9, 10};
   int left = 0, right = arr.size() - 1, target = 8;

   int answer = lower_bound(arr.begin(), arr.end(), target) - arr.begin();
   cout << answer;

   return 0;
}