Longest Substring with At Least K Repeating Characters

class Solution {
public:
int longestSubstring(string s, int k) {
if(s.size()<k || s.size()==0) return 0;
map<char,int> imap;
for(auto c:s){
if(imap.find(c)==imap.end()) imap[c] = 1;
else imap[c]++;
}
int idx = 0;
while(idx<s.size() && imap[s[idx]]>=k) idx++;
if(idx == s.size()) return s.size();

int left = longestSubstring(s.substr(0,idx),k);
int right = longestSubstring(s.substr(idx+1),k);
return max(left,right);
}
};