#include <iostream>
using namespace std;
#include <vector>
 
template <typename T>
std::vector<T> flatten(const std::vector<std::vector<T>> & vec) {   
    std::vector<T> result;
    for (const auto & v : vec)
        result.insert(result.end(), v.begin(), v.end());                                                                                         
    return result;
}
 
int main() {
	vector<vector<int>> v = {
		{ 8, 7, 6, 3 },
		{ 2, 1, 9, 4 },
		{ 1, 2, 3, 8 }
	};
	auto flat = flatten(v);
	for (auto i : flat)
		cout << i << " ";
	return 0;
}

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