非常好的two pointers的题; 下面是模版写法 (i作为右指针, left左)

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class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        HashMap<Character, Integer> map = new HashMap<>();
        int res = 0, left = 0;
        
        for (int i = 0; i < s.length(); ++i) {
            char c = s.charAt(i);
            map.put(c, map.getOrDefault(c, 0) + 1);
            
            while (map.size() > k) {
                char leftChar = s.charAt(left);
                map.put(leftChar, map.get(leftChar) - 1);
                if (map.get(leftChar) == 0) map.remove(leftChar);
                left++;
            }
            
            // update res
            res = Math.max(res, i - left + 1);
        }
        
        return res;
    }
}