Longest Substring with At Least K Repeating Characters

小鱼儿 2021-04-27 14:53 307阅读 0赞

## 题目要求 ##

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.

找出字符串中的最长子字符串，满足该子字符串中任何字符出现的次数都大于k。

## 思路和代码 ##

这是一个经典的分治法解决的问题，关键在于我们如何将这个问题分解为更小的子问题。反过来想，这个子字符串一定不包含什么元素呢？当一个元素出现的总次数小于k，那么该元素一定不在子字符串中，只需要将其作为分界点，分别找出左半部分和右半部分的满足条件的最长子字符串。

public int longestSubstring(String s, int k) {
            return longestSubstring(s, k, 0, s.length()-1);
        }
        
        public int longestSubstring(String s, int k, int left, int right) {
            if(left > right) {
                return 0;
            }
    
            int[] count = new int[26];
            for(int i = left ; i<=right ; i++) {
                count[s.charAt(i) - 'a']++;
            }
            int result = right - left + 1;
            for(int i = left ; i<=right ; i++) {
                if(count[s.charAt(i)-'a'] < k && count[s.charAt(i)-'a'] > 0) {
                    int leftLongest = longestSubstring(s, k, left, i-1);
                    int rightLongest = longestSubstring(s, k, i+1, right);
                    result = Math.max(leftLongest, rightLongest);
                    
                    break;
                }
            }
            return result;
        }