# Program to find the length of longest substring which has two distinct elements in Python

Suppose we have a string s, we have to find the length of the longest substring that contains at most 2 distinct characters.

So, if the input is like s = "xyzzy", then the output will be 4, as "yzzy" is the longest substring with at most 2 unique characters.

To solve this, we will follow these steps−

• start := 0

• c := a map

• ans := 0

• for end in range 0 to size of s, do

• c[s[end]] := c[s[end]] + 1

• while size of c > 2, do

• c[s[start]] := c[s[start]] - 1

• if c[s[start]] is 0, then

• delete c[s[start]]

• start := start + 1

• ans := maximum of ans and (end - start + 1)

• return ans

Let us see the following implementation to get better understanding −

## Example

Live Demo

class Solution:
def solve(self, s):
from collections import Counter
start = 0
c = Counter()
ans = 0
for end in range(len(s)):
c[s[end]] += 1
while len(c) > 2:
c[s[start]] -= 1
if not c[s[start]]:
del c[s[start]]
start += 1
ans = max(ans, end - start + 1)
return ans
ob = Solution()
s = "xyzzy"
print(ob.solve(s))

## Input

s = "xyzzy"

## Output

4