395. Longest Substring with At Least K Repeating Characters

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.

Solution1：Divide & Conquer

思路：
Time Complexity: O(N*log底3(N)) Space Complexity: O(N)

Solution2：Divide & Conquer

思路：
Time Complexity: O() Space Complexity: O()

Solution1 Code：

class Solution {
    public int longestSubstring(String s, int k) {
        if(s == null || s.length() == 0) {
            return 0;
        }
        
        int[] count = new int[26];
        for(int i = 0; i < s.length(); i++) {
            count[s.charAt(i) - 'a']++;
        }
        
        char gap_c = ' ';
        for(int i = 0; i < count.length; i++) {
            if(count[i] < k && count[i] > 0) {
                gap_c = (char)('a' + i);
                break;
            }
        }
        
        // terminal
        if(gap_c == ' ') {
            return s.length();
        }
        
        // divide
        String[] substrs = s.split("" + gap_c);
        int[] res = new int[substrs.length];
        
        for(int i = 0; i < substrs.length; i++) {
            res[i] = longestSubstring(substrs[i], k);
        }
        
        // conquer
        int max_len = 0;
        for(int i = 0; i < res.length; i++) {
            if(res[i] > max_len) {
                max_len = res[i];
            }
        }
        
        return max_len;
    }
}