LeetCode - 395 - Longest Substring with At Least K Repeating Characters

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.

找出一个子串，使得每个字母最少重复k次

分治，先统计最长的串，若满足条件便返回；否则，找到不满足条件字母的位置，分别计算他左右的子串。如此递归下去。

class Solution {
public:
    int longestSubstring(string s, int k) {
        if (k > s.length()) return 0;
        return solve(s, 0, s.length()-1, k);
    }
    
    int solve(string s, int st, int ed, int k) {
        if (ed - st + 1 < k) return 0;
        unordered_map m(26);
        for (int i = st; i <= ed; ++i) {
            m[s[i]-'a']++;
        }
        for (auto x: m) {
            if (x.second >= k) continue;
            for (int i = st; i <= ed; ++i) {
                if (s[i] == x.first + 'a') {
                    int le = solve(s, st, i-1, k);
                    int ri = solve(s, i+1, ed, k);
                    return max(le, ri);
                }
            }
        }
        return ed - st + 1;
    }
};