LeetCode #395 - Longest Substring with At Least K Repeating Characters-优快云博客

本文介绍了一种使用递归方法寻找字符串中最长子串的技术，该子串中的每个字符至少出现k次。通过实例展示了如何统计字符频率并依据频率拆分字符串以找到符合条件的最长子串。

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题目描述：

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.

求一个字符串的最长子字符串，保证这个子字符串中每个字符最少出现k次。这道题需要用递归解决，首先要统计字符串中每个字符出现的次数，当每个字符次数都大于等于k，要求的子字符串就是它本身，否则由于次数少于k次的字符不能出现在子字符串中，所以要以它分隔符将字符串拆分为多段字符串，然后利用递归依次求最长子字符串。

class Solution {
public:
    int longestSubstring(string s, int k) {
        if(s.size()==0) return 0;
        unordered_map<char,int> hash;
        for(int i=0;i<s.size();i++)
        {
            if(hash.count(s[i])==0) hash[s[i]]=1;
            else hash[s[i]]++;
        }
        char c='#';
        unordered_map<char,int>::iterator it;
        for(it=hash.begin();it!=hash.end();it++)
        {
            if((*it).second<k)
            {
                c=(*it).first;
                break;
            }
        }
        if(c=='#') return s.size();
        
        istringstream iss(s);
        string temp;
        vector<string> v;
        while(getline(iss,temp,c)) v.push_back(temp);
        int max_length=0;
        for(int i=0;i<v.size();i++)
            max_length=max(max_length,longestSubstring(v[i],k));
        return max_length;
    }
};