387. First Unique Character in a String 

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class Solution {
    public int firstUniqChar(String s) {
        int[] charToIndexArray = new int[26];
        Arrays.fill(charToIndexArray, -1);

        int[] charToCountArray = new int[26];
        for (int i = 0; i < s.length(); i++) {
            char c = s.charAt(i);
            int offset = c - 'a'; // 注意此处减去 a 而不是减去 0

            charToCountArray[offset]++;
            if (charToIndexArray[offset] == -1) {
                charToIndexArray[offset] = i;
            }
        }

        int minIndex = Integer.MAX_VALUE;
        for (int i = 0; i < charToCountArray.length; i++) {
            if (charToCountArray[i] == 1) {
                minIndex = Math.min(minIndex, charToIndexArray[i]);
            }
        }

        return minIndex == Integer.MAX_VALUE ? -1 : minIndex;
    }
}

References

387. First Unique Character in a String