共计 1003 个字符，预计需要花费 3 分钟才能阅读完成。
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;
}