Longest Substring with At Least K Repeating Characters (Leetcode 395)

Problem Link: https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/

class Solution:
    def longestSubstring(self, s: str, k: int) -> int:
        
        n=len(s)
        d={}
        for i in s:
            if(i in d):
                d[i]+=1
            else:
                d[i]=1
        
        for i in range(n):
            if(d[s[i]]<k):
                l=self.longestSubstring(s[:i],k)
                r=self.longestSubstring(s[i+1:],k)
                if(l>=r):
                    return l
                else:
                    return r
        else:
            return n