340. Longest Substring with At Most K Distinct Characters 最多k个字母不同的字符串

Given a string s and an integer k, return the length of the longest 

Example 1:

Input: s = "eceba", k = 2
Output: 3
Explanation: The substring is "ece" with length 3.

Example 2:

Input: s = "aa", k = 1
Output: 2
Explanation: The substring is "aa" with length 2.

public int lengthOfLongestSubstringKDistinct(String s, int k) {
    Map<Character, Integer> map = new HashMap<>();
    int left = 0;
    int best = 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);
            if (map.containsKey(leftChar)) {
                map.put(leftChar, map.get(leftChar) - 1);                     
                if (map.get(leftChar) == 0) { 
                    map.remove(leftChar);
                }
            }
            left++;
        }
        best = Math.max(best, i - left + 1);
    }
    return best;
}