From vector does not name a type To expected unqualified id / C++ brute force

Long story short. Im trying to run this code in VS Studio IDE.

#include <iostream>
#include <vector>

using std::vector;

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        for(int i = 0; i < nums.size(); i++){
            for(int j = i + 1; j < nums.size(); j++){
                if(nums[i] + nums[j] == target){
                    return {i, j};
                }
            }            
        }
        return {};
    }
};

I had to add #include to get the first problem resolved. But now its saying:

using-declaration for non-member at class scope

expected unqualified-id before '<' token

The plot thickens. It runs fine on leetcode without the #include . None of this transpires.

Is it a vector issue or what? And why does VS Studio code hate this but it runs fine on leetcode? https://leetcode.com/problems/two-sum/solutions/1439021/c-brute-force-hashmap-solutions/?q=brute&orderBy=most_votes&languageTags=cpp

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        for(int i = 0; i < nums.size(); i++){
            for(int j = i + 1; j < nums.size(); j++){
                if(nums[i] + nums[j] == target){
                    return {i, j};
                }
            }            
        }
        return {};
    }
};

OG code.

Thanks in advance.

Although I would NOT recommend the use of using at header level because of namespace clashes, if you really want to use it, consider the version below:

#include <iostream>
#include <vector>

using namespace std;

class Solution {
public:
    vector<int> twoSum(vector<int>& nums, int target) {
        for(int i = 0; i < nums.size(); i++){
            for(int j = i + 1; j < nums.size(); j++){
                if(nums[i] + nums[j] == target){
                    return {i, j};
                }
            }            
        }
        return {};
    }
};

That said, most people would agree that the approach below looks more "modern"

#include <iostream>
#include <vector>
#include <tuple>
#include <optional>

class Solution {
public:
    std::optional< std::tuple<int,int> > twoSum( const std::vector<int>& nums, int target) {
        for( int i = 0; i < nums.size(); i++) {
            for(int j = i + 1; j < nums.size(); j++) {
                if( nums[i] + nums[j] == target){
                    return {{i, j}};
                }
            }            
        }
        return {};
    }
};


int main() {
    std::vector<int> vec{1,2,3,4};
    int total = 5;
    Solution s;
    auto result = s.twoSum( vec, total );
    if ( result ) {
        auto [i,j] = result.value();
        std::cout << vec[i] << " + " << vec[j] << " = " << total << std::endl;
    }
}