Longest Substring with At Most Two(K) Distinct Characters

LC. 159 & 340

Given a string s, find the length of the longest substring t that contains at most 2 distinct characters.

Example 1:

Input: "eceba"
Output: 3
Explanation: "ece"

Example 2:

Input: "ccaabbb"
Output: 5
Explanation: "aabbb" 

My Solutions:

无论是找最多2个字母还是k个字母，都是用hashmap记录每个字母出现的字数。如果hashmap的长度超过了2个（k个），需要删掉一个entry，并且移动左坐标。

public int lengthOfLongestSubstringTwoDistinct(String s) {
    if (s == null || s.length() == 0) return 0;
    int maxlen = 0, j = 0;
    Map<Character, Integer> countMap = new HashMap<>();
    
    // i是不断移动的右坐标，j是需要移动时才移动的左坐标
    for (int i = 0; i < s.length(); i++) {
        char c = s.charAt(i);
        countMap.put(c, countMap.getOrDefault(c, 0) + 1);
        
        while (countMap.size() > 2 && j <= i) {
            char chj = s.charAt(j);
            // 把在j指针位置的字母个数减1或者删去，移动j指针
            if (countMap.get(chj) == 1) countMap.remove(chj);
            else countMap.put(chj, countMap.get(chj) - 1);
            j++;
        }
        // 更新最大值
        maxlen = Math.max(maxlen, i - j + 1);
    }
    return maxlen;
}