First unique character in a string

https://leetcode.com/problems/first-unique-character-in-a-string

Given a string, find the first non-repeating character in it and return it’s index. If it doesn’t exist, return -1.

Examples:
s = "leetcode"
return 0.
s = "loveleetcode",
return 2.

Note: You may assume the string contain only lowercase letters.

Implementation :

class Solution {
    public int firstUniqChar(String s) {
        HashMap<Character, Integer> count = new HashMap<Character, Integer>();
        int n = s.length();
        // build hash map : character and how often it appears
        for (int i = 0; i < n; i++) {
            char c = s.charAt(i);
            count.put(c, count.getOrDefault(c, 0) + 1);
        }
        // find the index
        for (int i = 0; i < n; i++) {
            if (count.get(s.charAt(i)) == 1) 
                return i;
        }
        return -1;
    }
}

References :

https://leetcode.com/articles/first-unique-character-in-a-string