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.

Solution:

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