Repeating Sub-Sequence

• Time : $O(n)$
• Space : $O(1)$

int Solution::anytwo(string A) {
    unordered_map<char, vector<int>> mp;
    for(int i = 0; i < A.length(); i++) mp[A[i]].push_back(i);
    for(char a = 'a'; a <= 'z'; a++) {
        for(char b = 'a'; b <= 'z'; b++) {
            if(mp[a].size() < 2) continue;
            if(mp[b].size() < 2) continue;
            if(a == b) {
                if(mp[a].size() >= 3) return true;
            } else {
                auto& A = mp[a];
                auto& B = mp[b];
                if(A[0] < B[B.size() - 2] and A[1] < B[B.size() - 1]) return true;
            }
        }
    }
    return 0;
}