Finding the palindrome index.cpp

const auto mid = next(begin(S), S.size()/2); // we'll reuse it
const auto diffIt = mismatch(begin(S), mid, rbegin(S)).first;
const auto diffIdx = distance(begin(S), diffIt); // diffIt as 0-based position

if (equal(next(diffIt), mid, next(rbegin(S), diffIdx))) 
  cout << diffIdx << endl;
else
  cout << S.size() - diffIdx - 1 << endl;