# 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.```

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## Python Solution

``````class Solution:
def lengthOfLongestSubstringTwoDistinct(self, s: str) -> int:
max_length = 0
visited = {}

i = 0
j = 0
while j < len(s):
c = s[j]

if c in visited:
visited[c] = visited[c] + 1
else:
visited[c] = 1

if len(visited) > 2:
max_length = max(max_length, j - i)

while len(visited) > 2:
t = s[i]
count = visited[t]
if count > 1:
visited[t] = count -1
else:
del visited[t]
i += 1
j += 1

return max(max_length, len(s) - i)
``````
• Time Complexity: ~N
• Space Complexity: ~1

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