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 = 3

Output:
3

The longest substring is "aaa", as 'a' is repeated 3 times.

Input:
s = "ababbc", k = 2

Output:
5

The 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 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;
    }
};