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 English letters.

class Solution {
    public int firstUniqChar(String s) {
        HashMap<Character, Integer> map = new HashMap<>();
        for (char c : s.toCharArray())
            map.put(c, map.getOrDefault(c, 0) + 1);
        int ans = -1;
        for (int i = 0; i < s.length(); i++) {
            if (map.get(s.charAt(i)) == 1) {
                ans = i;
                break;
            }
        }
        return ans;
    }
}