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.

这个题目挺不错了。

主要还是统计出现次数没有达到k的字母作为分界点，我写的递归类似线段树，一开始超时了，我也没搞清楚为什么，后来调试的时候发现for循环的时候重复计算了，加个return就过了。

笨拙的代码

class Solution {
public:
    int ans = 0;
    map<char,int> mp;
    void dfs(int l, int r, int k, string &s) {
        if (l > r || r - l + 1 < k) return ;
        int mark = 0;
        mp.clear();
        for (int i = l; i <= r; ++i) mp[s[i]]++;
        for (int i = l; i <= r; ++i) {
            if(mp[s[i]] < k) {
                dfs(l, i - 1, k, s);
                dfs(i + 1, r, k, s);
                return ;
                mark = 1;
            }
        }
        if (!mark) {
            ans = max(r - l + 1, ans);
        }
    }
    int longestSubstring(string s, int k) {
        int n = s.length();
        dfs(0, n - 1, k, s);
        return ans;
    }
};