395 Longest Substring with At Least K Repeating Characters

Leetcode 395: Longest Substring with At Least K Repeating Characters
Detailed Video Explanation: https://youtu.be/bHZkCAcj3dc
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C++:
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int longestSubstring(string s, int k) {
    int n = s.length();
    if(n == 0 || n < k) return 0;
    if(k <= 1) return n;

    unordered_map<char, int> counts;
    for(char c: s) counts[c] += 1;

    int l = 0;
    while(l < n && counts[s[l]] >= k) l++;
    if(l >= n-1) return l;

    int ls1 = longestSubstring(s.substr(0, l), k);
    while(l < n && counts[s[l]] < k) l++;
    int ls2 = (l < n) ? longestSubstring(s.substr(l), k) : 0;
    return max(ls1, ls2);
}


Java:
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public int longestSubstring(String s, int k) {
    int n = s.length();
    if(n == 0 || n < k) return 0;
    if(k <= 1) return n;

    Map<Character, Integer> counts = new HashMap();
    for(char c: s.toCharArray()) 
        counts.put(c, counts.getOrDefault(c, 0) + 1);

    int l = 0;
    while(l < n && counts.get(s.charAt(l)) >= k) l++;
    if(l >= n-1) return l;

    int ls1 = longestSubstring(s.substring(0, l), k);
    while(l < n && counts.get(s.charAt(l)) < k) l++;
    int ls2 = (l < n) ? longestSubstring(s.substring(l), k) : 0;
    return Math.max(ls1, ls2);
}


Python3:
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def longestSubstring(self, s: str, k: int) -> int:
    n = len(s)
    if n == 0 or n < k: return 0
    if k <= 1: return n

    counts = Counter(s)

    l = 0
    while l < n and counts[s[l]] >= k: l += 1
    if l >= n-1: return l

    ls1 = self.longestSubstring(s[0:l], k)
    while l < n and counts[s[l]] < k: l += 1
    ls2 = self.longestSubstring(s[l:], k) if l<n else 0
    return max(ls1, ls2)