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.

class Solution {
    public int firstUniqChar(String s) {
        Map<Character, Integer> hash = new HashMap<>();
        if (s == null || s.length() == 0) return -1;

        for (char c : s.toCharArray()){
            hash.put(c, hash.getOrDefault(c, 0) + 1);
        }

        for (int i = 0; i < s.length(); i++){
            if ((hash.get(s.charAt(i))) == 1){
                return i;
            }
        }

        return -1;
    }
}