# LeetCode 159. Longest Substring with At Most Two Distinct Characters

## Description

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

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.```

## Explanation

Two pointers. Keep faster pointer moving to the right until more than 2 distinct characters. Then adjust slower pointer and repeat the same.

## Python Solution

``````class Solution:
def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
if not s:
return ""

max_length = 1
j = 0
counter = {}

for i in range(len(s)):
while j < len(s) and (len(counter) < 2 or (len(counter) == 2 and s[j] in counter)):
counter[s[j]] = counter.get(s[j], 0) + 1
j += 1

if len(counter) <= 2:
max_length = max(max_length, j - i)

counter[s[i]] -= 1

if counter[s[i]] == 0:
del counter[s[i]]

return max_length
``````
• Time Complexity: ~N
• Space Complexity: ~1

where N is a number of characters in the input string.