View Compilation Error

0_0_25164071_28630.cpp: In function 'int main()':
0_0_25164071_28630.cpp:69:6: error: 'class std::set<int>' has no member named 'push_back'
    a.push_back(tmp);
      ^
0_0_25164071_28630.cpp:77:19: error: no match for 'operator[]' (operand types are 'std::set<int>' and 'int')
     tmp = change(a[k], i, j);
                   ^