You have typo in your code and it did not work, It should have been '\0' instead of '\n'.

So the following fix works.

string to_postfix(const string& infix) {
  int pos = find(infix.begin(), infix.end(), '\0') - infix.begin();
  return to_postfix_NEW(infix.substr(0, pos)) + infix.substr(pos, infix.size() - pos);
}

simpleAndNestedLoops(SolutionTests)
 Your interpreter should jump to the command straight *after* the matching "[" on the iteration where it hits the "]" and *not* the matching "[" itself expected:<10000
01000
00[1]00
00000
00000
00...> but was:<10000
01000
00[0]00
00000
00000
00...>

I think the spec for ']' differs from the one of this test case. My solution is in Java.

Line if (token.length()>1) mPlus.remove(mPlus.size()-1);
is redundant and can be removed.

How do I handle "Unknown Error"? Help please!

Add Rust too, please!