# 0159. Longest Substring with At Most Two Distinct Characters

<https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters>

## Description

Given a string `s`, return *the length of the longest substring that contains at most **two distinct characters***.

**Example 1:**

```
**Input:** s = "eceba"
**Output:** 3
**Explanation:** The substring is "ece" which its length is 3.
```

**Example 2:**

```
**Input:** s = "ccaabbb"
**Output:** 5
**Explanation:** The substring is "aabbb" which its length is 5.
```

**Constraints:**

* `1 <= s.length <= 105`
* `s` consists of English letters.

## ac: sliding window

same as this one: <https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/description/>

```java
class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        // edge cases
        if (s == null || s.length() == 0) return 0;

        // build note
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        int l = 0, r = 0, lgst = 0, cnt = 0;

        // iterate
        for (; r < s.length(); r++) {
            char c = s.charAt(r);
            map.put(c, map.getOrDefault(c, 0) + 1);
            if (map.get(c) == 1) cnt++;  // new character appear in current window

            while (cnt > 2) {  // one window formed
                lgst = Math.max(lgst, r - l);  // store length
                char lc = s.charAt(l);
                map.put(lc, map.get(lc) - 1);
                l++;  // remove left char, move window

                if (map.get(lc) == 0) cnt--;  // reduce one char count 
            }
        }
        lgst = Math.max(lgst, r - l);  // update last window value

        // result
        return lgst;
    }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://jaywin.gitbook.io/leetcode/solutions/0159-longest-substring-with-at-most-two-distinct-characters.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.