# Longest Substring with At Least K Repeating Characters in C++

Suppose we have a string s, and we have to find the length of the longest substring T of that given string (consists of lowercase letters only) such that every character in T appears no less than k times. So if the string is “ababbc” and k = 2, then the output will be 3 and longest substring will be “ababb”, as there are two a’s and three b’s.

To solve this, we will follow these steps −

• create one recursive function called longestSubstring(), this takes string s and size k
• if k = 1, then return the size of the string
• if size of string < k, then return 0
• create one array c of size 26, and fill this with -1
• n := size of the string
• for i in range 0 to n
• if c[s[i] – ‘a’] = -1, then c[s[i] – ‘a’] := 1, otherwise increase c[s[i] – ‘a’] by 1
• for i in range 0 to 25
• if c[i] is not -1 and c[i] < k, then badChar := i + ‘a’ and come out from the loop
• if badChar = ‘*’, then return n
• v := another array of strings by splitting the original string using badChar
• ans := 0
• for i in range 0 to size of v
• ans := max of ans and longestSubstring(v[i], k)
• return ans

## Example(C++)

Let us see the following implementation to get better understanding −

Live Demo

#include <bits/stdc++.h>
using namespace std;
class Solution {
public:
vector <string> splitString(string s, char x){
string temp = "";
vector <string> res;
for(int i = 0; i < s.size(); i++){
if(s[i] == x){
if(temp.size())res.push_back(temp);
temp = "";
}
else temp += s[i];
}
if(temp.size())res.push_back(temp);
return res;
}
int longestSubstring(string s, int k) {
if(k == 1)return s.size();
if(s.size() < k )return 0;
vector <int> cnt(26, -1);
int n = s.size();
for(int i = 0; i < n; i++){
if(cnt[s[i] - 'a'] == -1) cnt[s[i] - 'a'] = 1;
else cnt[s[i] - 'a']++;
}
for(int i = 0; i < 26; i++){
if(cnt[i] != -1 && cnt[i] < k){
break;
}
}
vector <string> xx = splitString(s, badChar);
int ans = 0;
for(int i = 0; i < xx.size(); i++)ans = max(ans, longestSubstring(xx[i], k));
return ans;
}
};
main(){
Solution ob;
cout << ob.longestSubstring("ababbc", 2);
}

## Input

"ababbc"
2

## Output

5