README.md

1930. Unique Length-3 Palindromic Subsequences

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 (Rust)

1. Solution

impl Solution {
    pub fn count_palindromic_subsequence(s: String) -> i32 {
        let s = s.as_bytes();
        let mut ret = 0;

        for c in b'a'..=b'z' {
            let mut mask = 0_i32;

            for i in s.iter().position(|&x| x == c).unwrap_or(0) + 1
                ..s.iter().rposition(|&x| x == c).unwrap_or(0)
            {
                mask |= 1 << (s[i] - b'a');
            }

            ret += mask.count_ones();
        }

        ret as i32
    }
}