leetcode + DFS题

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class Solution {
public:
    int helper(string s, int k, int left, int right)
    {
        int len = right-left+1;
        if(len<=0) return 0;
        int i=0, j=0;
        int maxlen = 0;
        vector<int> count(26, 0);
        for(i= left; i<=right; i++){
            count[s[i]-'a'] +=1;
        }
        for(i=left, j=left; i<=right; i++){
            if(count[s[i]-'a']<k){
                maxlen = max(maxlen, helper(s, k, j, i-1));
                j = i+1;
            }
        }
        if(j==left) return len;
        else return max(maxlen, helper(s, k, j, i-1));
    }
    int longestSubstring(string s, int k){
        return helper(s, k, 0,s.size()-1);
    }
};