README_EN.md

1930. Unique Length-3 Palindromic Subsequences

中文文档

Description

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.

• For example, "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 <= 105
• s consists of only lowercase English letters.

Solutions

Python3

class Solution:
    def countPalindromicSubsequence(self, s: str) -> int:
        res = 0
        for i in range(26):
            c = chr(ord('a') + i)
            if c in s:
                l, r = s.index(c), s.rindex(c)
                chars = {s[j] for j in range(l + 1, r)}
                res += len(chars)
        return res

Java

class Solution {
    public int countPalindromicSubsequence(String s) {
        int res = 0;
        for (char c = 'a'; c <= 'z'; ++c) {
            int l = s.indexOf(c), r = s.lastIndexOf(c);
            Set<Character> chars = new HashSet<>();
            for (int i = l + 1; i < r; ++i) {
                chars.add(s.charAt(i));
            }
            res += chars.size();
        }
        return res;
    }
}

C++

class Solution {
public:
    int countPalindromicSubsequence(string s) {
        int res = 0;
        for (char c = 'a'; c <= 'z'; ++c) {
            int l = s.find_first_of(c), r = s.find_last_of(c);
            unordered_set<char> chars;
            for (int i = l + 1; i < r; ++i) {
                chars.insert(s[i]);
            }
            res += chars.size();
        }
        return res;
    }
};

Go

func countPalindromicSubsequence(s string) int {
	res := 0
	for c := 'a'; c <= 'z'; c++ {
		l, r := strings.Index(s, string(c)), strings.LastIndex(s, string(c))
		chars := make(map[byte]bool)
		for i := l + 1; i < r; i++ {
			chars[s[i]] = true
		}
		res += len(chars)
	}
	return res
}

...