395. Longest Substring with At Least K Repeating Characters

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

Problem

Find the length of the longest substring T of a given string (consists of lowercase letters only) such that every character in T appears no less than k times.

Example 1:

Input:
s = "aaabb", k = 3

Output:
3

The longest substring is "aaa", as 'a' is repeated 3 times.

Example 2:

Input:
s = "ababbc", k = 2

Output:
5

The longest substring is "ababb", as 'a' is repeated 2 times and 'b' is repeated 3 times.

Solution

class Solution {
public:
    int longestSubstring(const string &s, const int k) {
        if (s.empty() || s.size() < k) return 0;
        return longestSubstring(s, k, 0, s.size() - 1);
    }
    int longestSubstring(const string &s, const int k, int begin, int end) {
        const int n = end - begin + 1;
        if (n < k) return 0;
        int counter[26] = {0};
        for (int i = begin; i <= end; ++i) ++counter[s[i] - 'a'];
        bool found = true;
        for (int i = begin; i <= end; ++i) {
            if (counter[s[i] - 'a'] < k) {
                found = false; break;
            }
        }
        if (found) return n;
        // found = false;
        int l = 0, r = 0;
        int max_len = 0;
        for (int i = begin; i <= end; ++i) {
            if (counter[s[i] - 'a'] < k) continue;
            if (!found) {
                found = true;
                l = r = i;
            } else {
                r = i;
            }
            if (i == end || counter[s[i + 1] - 'a'] < k) {
                max_len = max(max_len, longestSubstring(s, k, l, r));
                found = false;
            }
        }
        return max_len;
    }
};

• #recursive

• #string