Longest K unique characters substring

Problem Link: https://practice.geeksforgeeks.org/problems/longest-k-unique-characters-substring0853/1

class Solution:
    def longestKSubstr(self, s, k):
        # code here
        c=0
        j=0
        ans=0
        d={}
        n=len(s)
        for i in range(n):
            if(s[i] not in d):
                d[s[i]]=1
            else:
                d[s[i]]+=1
            
            while(len(d)>k):
                if(d[s[j]]>1):
                    d[s[j]]-=1
                    j+=1
                else:
                    del d[s[j]]
                    j+=1
            ans=max(ans,i-j+1)
            
            ### If k is larger than 
            ### the size of dictionary
            if(i-j+1==n and k>len(d)):
                ans=-1
            
        return ans