HashMap

Without Repeating Characters: as a result, Integer value in HashMap would better keep record of the index where this character occur than freq. Two pointers should always increase. 

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class Solution {
    public int lengthOfLongestSubstring(String s) {
        HashMap<Character, Integer> map = new HashMap<>();
        int i, j = 0, res = 0;
        for (i = 0; i < s.length(); ++i) {
            if (map.containsKey(s.charAt(i))) {
                j = Math.max(j, map.get(s.charAt(i)) + 1);
            }
            map.put(s.charAt(i), i);
            res = Math.max(res, i - j + 1);
        }
        return res;
    }
}

HashSet or int[256] for mapping are both possible but I am too lazy to write them.