LeetCode Entry

1930. Unique Length-3 Palindromic Subsequences

21.11.2025 medium 2025 kotlin rust

palindroms subsequences

1930. Unique Length-3 Palindromic Subsequences medium blog post substack youtube

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Problem TLDR

3-palindroms subsequences #medium

Intuition

    // how many pairs we have for alphabet?
    // 26*26 - 500*10^5 = 10^7 too big
    // a b c d a  so, between same chars every uniq counts
    // a......b........a.......b  can intersect

Approach

• we can use bitmask for speedup

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(1)\)

Code

// 121ms
    fun countPalindromicSubsequence(s: String) =
    ('a'..'z').sumOf { s.slice(s.indexOf(it)+1..<s.lastIndexOf(it)).toSet().size }

// 127ms
    pub fn count_palindromic_subsequence(s: String) -> i32 {
        ('a'..='z').filter_map(|c| {
            let r = s.rfind(c)?; let l = s[..r].find(c)?;
            Some(s[l+1..r].chars().unique().count() as i32)
        }).sum()
    }