Substrings with same first and last characters

import java.util.*;

class Solution {
    public int countSubstring(String s) {
        int n = s.length();
        int count = 0;
        HashMap<Character, Integer> ans = new HashMap<>();
        for (int i = 0; i < n; i++) {
            char ch = s.charAt(i);
            ans.put(ch, ans.getOrDefault(ch, 0) + 1);
            count += ans.get(ch);
        }
        return count;
    }
}