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 ;
         if (s.length() <= k) return s.length();

         int begin = , end = ;
         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), ) + );
             while (map.size() > k) {
                 if (map.get(s.charAt(begin)) == ) {
                     map.remove(s.charAt(begin));
                 } else {
                     map.put(s.charAt(begin), map.get(s.charAt(begin)) - );
                 }
                 begin++;
             }
             tempMaxLength = Math.max(tempMaxLength, end - begin + );
             end++;
         }
         return tempMaxLength;
     }
 }