395. Longest Substring with At Least K Repeating Characters

https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/discuss/87739/Java-Strict-O(N)-Two-Pointer-Solution

window分别为1-26个unique的字母，找出为这个长度的字母的 其中至少有K repeating ones的最长子序列，取最大值

 

 

class Solution {
    public int longestSubstring(String s, int k) {
        if(s.length() < k) return 0;
        int max = 0;
        for(int i = 0; i < 26; i++){
            int[] arr = new int[26];
            int unique = 0;
            int lo = 0, hi = 0;
            while(hi < s.length()){
                boolean valid = true;
                if(arr[s.charAt(hi) - 'a']++ == 0) unique++;
                hi++;
                while(unique > i){  //搞到unique == i
                    if(arr[s.charAt(lo++) - 'a']-- == 1) unique--;
                }
                for(int j = 0; j < 26; j++){   //看看存不存在出现但是不到k次数的字母
                    if(arr[j] > 0 && arr[j] < k){  //注意arr[j] > 0 因为要出现
                        valid = false;
                        break;
                    }
                }
                if(unique == i && valid){ 
                    max = Math.max(max, hi-lo); //hi-lo
                }
                
            }   
        }
        return max;   
    }
}