class Solution {
    public int lengthOfLongestSubstring(String s) {

        int len = s.length(), max = 0;
        Map<Character, Integer> map = new HashMap<>();
        //try to extend the range [start, end]
        int start = 0;
        for (int end = 0; end <len; end++) {
            if (map.containsKey(s.charAt(end))) {
                start = Math.max(map.get(s.charAt(end))+1,start);
            }
            max = Math.max(max, end - start + 1);
            map.put(s.charAt(end), end);
        }
        return max;
    }
}