340. Longest Substring with At Most K Distinct Characters

Given a string, find the longest substring that contains only two unique characters. For example, given "abcbbbbcccbdddadacb", the longest substring that contains k unique character is "bcbbbbcccb".

分析：

用hashmap记录每个character从start到当前位置出现的次数，如果第k + 1个character出现, 更新maxLength，我们需要把左边的pointer(start) 往右移，直到从start到current之间只有K个character.

public class Solution {
    public int lengthOfLongestSubstringKDistinct(String s, int k) {
        if (s == null) return 0;
        if (s.length() <= k) return s.length();
        
        int begin = 0, end = 0;
        Map<Character, Integer> map = new HashMap<>();
        int tempMaxLength = Integer.MIN_VALUE;
        
        while (end < s.length()) {
            map.put(s.charAt(end), map.getOrDefault(s.charAt(end), 0) + 1);
            while (map.size() > k) {
                if (map.get(s.charAt(begin)) == 1) {
                    map.remove(s.charAt(begin));
                } else {
                    map.put(s.charAt(begin), map.get(s.charAt(begin)) - 1);
                }
                begin++;
            }
            tempMaxLength = Math.max(tempMaxLength, end - begin + 1);
            end++;
        }
        return tempMaxLength;
    }
}