LeetCode Entry

3637. Trionic Array I

03.02.2026 easy 2026 kotlin rust

Validate inc,dec,inc sequency

3637. Trionic Array I easy blog post substack youtube

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Problem TLDR

Validate inc,dec,inc sequency #easy

Intuition

Brute-Force O(n^3) is accepted.

Approach

• we can count peaks
• we can convert to string and use regex
• chunk_by, eq in Rust

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(1)\)

Code

// 32ms
    fun isTrionic(n: IntArray) =
    n.map{it}.zipWithNext(Int::compareTo)
        .joinToString("").matches(Regex("(-1)+1+(-1)+"))
    /*
    n.map{it}.zipWithNext(Int::compareTo).let { s ->
        s[0]<0 && 0 !in s && s.windowed(2).count{(a,b)->a!=b}==2 }

    n[0]<n[1] && 2 == (2..<n.size).count {
       (n[it-2]<n[it-1])!=(n[it-1]<n[it]) } &&
       (1..<n.size).all{n[it]!=n[it-1]}
    */

// 0ms
    pub fn is_trionic(n: Vec<i32>) -> bool {
        n.windows(2).map(|w|w[1].cmp(&w[0])as i8).collect::<Vec<_>>()
        .chunk_by(i8::eq).map(|c|c[0]).eq([1,-1,1])
    }