c++ solution without erasing or adding stuff to data structures - using previous left and right (pl and pr) indexes and an array to store elements that have been "erased" (used vector).

string superReducedString(string s) { int n = s.size(); vector used(n,false); int pl = -1, pr = -1;

int i = 0;
while (i < n) {
    if (i+1 < n and s[i]==s[i+1]) {
        int l = !used[i] ? i : pl;
        int r = !used[i+1] ? i+1 : pr;

        while (l >= 0 and r < n) {
            if (s[l]!=s[r]) break;

            used[l]=used[r]=true;

            while (l>=0 and used[l]) l--;
            while (r<n and used[r]) r++;
        }
        pl = l, pr = r;
    }
    i=max(i+1,pr);
}