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.

分析：

首先统计每个字母出现的次数，若该词出现的次数大于0且小于k的话，则其肯定不在最后的字符串中，即最大串出现在其左边或是右边。因此可以将问题分成2个子问题。

public class Solution {
    public int longestSubstring(String s, int k) {
        int n=s.length();
        return subHelp(s,k,0,n-1);
    }
    
    public int subHelp(String s,int k,int start,int end){
    	int n=end-start+1;
    	if(n<k)
    	    return 0;
    	int[] alpha=new int[26];
    	for(int i=start;i<=end;i++){
        	alpha[s.charAt(i)-'a']++;
        }
    	
    	int max=0,index;
    	int max1=0,max2=0;
    	for(int i=0;i<26;i++){
    		if(alpha[i]>0&&alpha[i]<k){
    		    index=s.indexOf(i+'a',start);
    			if((index-1)>start)
    				max1=subHelp(s,k,start,index-1);
    			if((index+1)<end)
    				max2=subHelp(s,k,index+1,end);
    			max=max1>max2?max1:max2;
    			return max;
    		}
    	}
    
    	return n;
    }
}