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# Longest Substring with At Most Two Distinct Characters

**Hard**

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: t is "ece" which its length is 3.
```

**Example 2:**

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

**Related Problems**

[Longest Substring with At Most K Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/)

## Analysis

Same as [Longest Substring with At Most K Distinct Characters](https://leetcode.com/problems/longest-substring-with-at-most-k-distinct-characters/), when k = 2.

Sliding window two pointer technique.

## Solution

Sliding Window - Two Pointers (15 ms, faster than 22.97%)

```java
class Solution {
    public int lengthOfLongestSubstringTwoDistinct(String s) {
        if (s == null || s.length() == 0) {
            return 0;
        }
        char[] chs = s.toCharArray();
        int n = s.length();

        HashMap<Character, Integer> count = new HashMap<>();

        int longest = 0;

        int firstIdx = 0;

        for (int i = 0; i < n; i++) {
            count.put(chs[i], count.getOrDefault(chs[i], 0) + 1);
            while (count.size() > 2) {
                count.put(chs[firstIdx], count.get(chs[firstIdx]) - 1);
                if (count.get(chs[firstIdx]) == 0) {
                    count.remove(chs[firstIdx]);
                }
                firstIdx++;
            }
            longest = Math.max(longest, i - firstIdx + 1);
        }
        return longest;
    }
}
```

## Reference

<https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/discuss/49708/Sliding-Window-algorithm-template-to-solve-all-the-Leetcode-substring-search-problem>.