Given a string s, return the number of unique palindromes of length three that are a subsequence of s.
Note that even if there are multiple ways to obtain the same subsequence, it is still only counted once.
A palindrome is a string that reads the same forwards and backwards.
A subsequence of a string is a new string generated from the original string with some characters (can be none) deleted without changing the relative order of the remaining characters.
"ace" is a subsequence of "abcde".
Example 1:
Input: s = "aabca" Output: 3 Explanation: The 3 palindromic subsequences of length 3 are: - "aba" (subsequence of "aabca") - "aaa" (subsequence of "aabca") - "aca" (subsequence of "aabca")
Example 2:
Input: s = "adc" Output: 0 Explanation: There are no palindromic subsequences of length 3 in "adc".
Example 3:
Input: s = "bbcbaba" Output: 4 Explanation: The 4 palindromic subsequences of length 3 are: - "bbb" (subsequence of "bbcbaba") - "bcb" (subsequence of "bbcbaba") - "bab" (subsequence of "bbcbaba") - "aba" (subsequence of "bbcbaba")
Constraints:
3 <= s.length <= 105s consists of only lowercase English letters.class Solution {
public int countPalindromicSubsequence(String s) {
int[] first = new int[26], last = new int[26];
int n = s.length(), ans = 0;
Arrays.fill(first, -1);
for (int i = 0; i < n; i++) {
int c = s.charAt(i) - 'a';
if (first[c] == -1)
first[c] = i;
last[c] = i;
}
for (int i = 0; i < 26; i++) {
if (first[i] == -1)
continue;
Set<Character> set = new HashSet<>();
for (int j = first[i] + 1; j < last[i]; j++)
set.add(s.charAt(j));
ans += set.size();
}
return ans;
}
}