# Longest Substring with atmost two distinct characters

Problem Link 1: <https://leetcode.com/problems/longest-substring-with-at-most-two-distinct-characters/>

Problem Link 2: <https://www.lintcode.com/problem/longest-substring-with-at-most-two-distinct-characters/description>

```python
class Solution:
    """
    @param s: a string
    @return: the length of the longest substring T that contains at most 2 distinct characters
    """
    def length_of_longest_substring_two_distinct(self, s: str) -> int:
        # Write your code here
        d={}
        j=0
        ans=0
        n=len(s)
        for i in range(n):
            if(s[i] in d):
                d[s[i]]+=1
            else:
                d[s[i]]=1
            
            while(len(d)>2):
                if(d[s[j]]==1):
                    del d[s[j]]
                    j+=1
                else:
                    d[s[j]]-=1
                    j+=1
            
            ans=max(ans,i-j+1)
        return ans
```
