First Unique Character in a String

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

Examples:

s = "leetcode"
return 0.

s = "loveleetcode"
return 2.

Note: You may assume the string contains only lowercase English letters.

Solution:

class Solution {
    public int firstUniqChar(String s) {
        int[] map = new int[26];
        for (char c : s.toCharArray()) {
            map[c - 'a'] ++;
        }
        for (int i = 0; i < s.length(); i ++) {
            char c = s.charAt(i);
            if (map[c - 'a'] == 1) {
                return i;
            }
        }
        return -1;
    }
}