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 = 3Output:
3The longest substring is "aaa", as 'a' is repeated 3 times.
Example 2:Input:
s = "ababbc", k = 2Output:
5The 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;}