Compilare: user.cpp: In function ‘bool back(int, int, std::vector<std::pair<int, int> >&, std::vector<std::vector<bool> >&)’: user.cpp:10:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < matrix.size(); ++i) { ~~^~~~~~~~~~~~~~~ user.cpp:11:27: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int j = 0; j < matrix[0].size(); ++j) { ~~^~~~~~~~~~~~~~~~~~ user.cpp:12:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (j < matrix[0].size() - 2 && !matrix[i][j] && matrix[i][j + 1] && matrix[i][j + 2]) { ~~^~~~~~~~~~~~~~~~~~~~~~ user.cpp:38:19: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] if (i < matrix.size() - 2 && !matrix[i][j] && matrix[i + 1][j] && matrix[i + 2][j]) { ~~^~~~~~~~~~~~~~~~~~~ user.cpp: In function ‘int main()’: user.cpp:84:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < heroes; ++i) { ~~^~~~~~~~ user.cpp:93:23: warning: comparison between signed and unsigned integer expressions [-Wsign-compare] for (int i = 0; i < solution.size(); i += 2) { ~~^~~~~~~~~~~~~~~~~