LeetCode395. Longest Substring with At Least K Repeating Characters

一、题目

Given a string s and an integer k, return the length of the longest substring of s such that the frequency of each character in this substring is greater than or equal to k.

if no such substring exists, return 0.

Example 1:

Input: s = “aaabb”, k = 3
Output: 3
Explanation: The longest substring is “aaa”, as ‘a’ is repeated 3 times.
Example 2:

Input: s = “ababbc”, k = 2
Output: 5
Explanation: The longest substring is “ababb”, as ‘a’ is repeated 2 times and ‘b’ is repeated 3 times.

Constraints:

1 <= s.length <= 104
s consists of only lowercase English letters.
1 <= k <= 105

二、题解

枚举只包含1种字符，满足大于等于k个重复字符的最大字符长度。再枚举包含2种字符，满足大于等于k个重复字符的最大字符长度。一直到包含26种字符，满足大于等于k个重复字符的最大字符长度。

class Solution {
public:
    int longestSubstring(string s, int k) {
        int res = 0,n = s.size();
        unordered_map<char,int> cnt;
        //枚举1-26的情况
        for(int req = 1;req <= 26;req++){
            cnt.clear();
            for(int l = 0,r = 0,satisfy = 0,collect = 0;r < n;r++){
                cnt[s[r]]++;
                if(cnt[s[r]] == 1) collect++;
                if(cnt[s[r]] == k) satisfy++;
                while(collect > req){
                    if(cnt[s[l]] == 1) collect--;
                    if(cnt[s[l]] == k) satisfy--;
                    cnt[s[l++]]--;
                }
                if(satisfy == req) res = max(res,r - l + 1);
            }
        }
        return res;
    }
};