What to assign a variable that I don't want to be equal to any possible input?

The question is

Implement the function unique_in_order which takes as argument a sequence and returns a list of items without any elements with the same value next to each other and preserving the original order of elements.

For example:

uniqueInOrder("AAAABBBCCDAABBB") == {'A', 'B', 'C', 'D', 'A', 'B'}

uniqueInOrder("ABBCcAD") == {'A', 'B', 'C', 'c', 'A', 'D'}

uniqueInOrder([1,2,2,3,3]) == {1,2,3}

Now my solution is

template <typename T> 
std::vector<T> uniqueInOrder(const std::vector<T>& iterable){
  std::vector<T> unique_set;
  T last = 0;
  for(auto & element : iterable) {
    if(element != last) {
      unique_set.push_back(element);
    }
    last = element;
  }
  return unique_set;
}
std::vector<char> uniqueInOrder(const std::string& iterable){
  std::vector<char> unique_set;
  char last = 0;
  for(auto & element : iterable) {
    if(element != last) {
      unique_set.push_back(element);
    }
    last = element;
  }
  return unique_set;
}

The problem is that sometimes the first element is 0. What can I assign last that will never match an input? I tried using NULL but I think that just compiled to 0 anyway.