LeetCode 395. Longest Substring with At Least K Repeating Characters

原题链接在这里：https://leetcode.com/problems/longest-substring-with-at-least-k-repeating-characters/

题目：

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.

题解：

To make sure every char in substring appears no less than k time, we need to make sure unique count == no less than k count.

We could add one more argument uniTargetCnt, that is the target of count of unique char.

First move the runner. 

While unique count > target, move the walker.

Update the longest with target unique count.

target unique count could be [1, 26].

Time Complexity: O(n). n = s.length().

Space: O(1).

AC Java:

 1 class Solution {
 2     public int longestSubstring(String s, int k) {
 3         if(s == null || s.length() == 0 || k <= 0){
 4             return 0;
 5         }
 6         
 7         int res = 0;
 8         for(int i = 1; i <= 26; i++){
 9             res = Math.max(res, longestTargetSubstring(s, k, i));
10         }
11         
12         return res;
13     }
14     
15     private int longestTargetSubstring(String s, int k, int uniTargetCnt){
16         int walker = 0;
17         int runner = 0;
18         int [] map = new int[26];
19         int uniCnt = 0;
20         int overKCnt = 0;
21         int res = 0;
22         
23         while(runner < s.length()){
24             if(map[s.charAt(runner) - 'a']++ == 0){
25                 uniCnt++;
26             }
27             
28             if(map[s.charAt(runner++) - 'a'] == k){
29                 overKCnt++;
30             }
31             
32             while(uniCnt > uniTargetCnt){
33                 if(map[s.charAt(walker) - 'a']-- == k){
34                     overKCnt--;
35                 }
36                 
37                 if(map[s.charAt(walker++) - 'a'] == 0){
38                     uniCnt--;
39                 }
40             }
41             
42             if(uniCnt == uniTargetCnt && uniCnt == overKCnt){
43                 res = Math.max(res, runner - walker);
44             }
45         }
46         
47         return res;
48     }
49 }

类似Minimum Window Substring.