Longest Substring with At Least K Repeating Characters问题及解法

问题描述：

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.

示例:

Input:s = "aaabb", k = 3Output:3The longest substring is "aaa", as 'a' is repeated 3 times.

Input:s = "ababbc", k = 2Output:5The longest substring is "ababb", as 'a' is repeated 2 times and 'b' is repeated 3 times.

问题分析：

这里注意关键一点，每个字符出现的总次数若是小于k，就肯定不会出现在答案中。所以我们采用分治算法求解该问题。

过程详见代码：

class Solution {public:    int longestSubstring(string s, int k) {        return find_longest(s, k, 0, (int)s.size());    }    int find_longest(string s, int k, int start, int end) {        vector<int> cnt(26, 0);        for (int i = start; i < end; ++i) ++cnt[s[i] - 'a'];        int mx = 0;        for (int i = start, j = i; i < end; i = j) {            while (i < end && cnt[s[i] - 'a'] < k) ++i;            if (i == end) break;            j = i + 1;            while (j < end && cnt[s[j] - 'a'] >= k) ++j;            if (i == start && j == end) return j - i;            mx = max(mx, find_longest(s, k, i, j));        }        return mx;    }};