159. Longest Substring with At Most Two Distinct Characters

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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: tis "ece" which its length is 3.

Example 2:

Input: "ccaabbb"
Output: 5
Explanation: tis "aabbb" which its length is 5.

Solution

public int lengthOfLongestSubstringTwoDistinct(String s) {
        HashMap<Character, Integer> countRecord = new HashMap<>();
        int startIndex = 0, maxLength = 0;
        for (int i = 0; i < s.length(); i++) {
            countRecord.put(s.charAt(i), countRecord.getOrDefault(s.charAt(i), 0) + 1);
            //Remeber 改變startIndex的判斷要用while
            while (countRecord.size() > 2) {
                //always keep the hashMap only has two set for 2 distinct characters
                //move the startIndex to next one, and decease the count value in map accordingly.
                countRecord.put(s.charAt(startIndex), countRecord.get(s.charAt(startIndex) - 1));
                if (countRecord.get(s.charAt(startIndex)) == 0) {
                    //remove the set if the count is 0
                    countRecord.remove(s.charAt(startIndex));
                }
                startIndex++;
            }
            maxLength = Math.max(maxLength, i - startIndex + 1);
        }
        return maxLength;
    }