LeetCode Entry

1291. Sequential Digits

13.07.2026 medium 2026 kotlin rust

Increasing digit numbers in a range

1291. Sequential Digits medium substack youtube

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Problem TLDR

Increasing digit numbers in a range

Intuition

The possible set of numbers is very small. Generate all then filter and sort.

Approach

• iterate over lengths, sliding window over string in an inner loop

Complexity

• Time complexity: \(O(log^2(n))\)

• Space complexity: \(O(log^2(n))\)

Code

    fun sequentialDigits(l: Int, h: Int) =
    (2..9).flatMap{"123456789".windowed(it)}.map{it.toInt()}.filter{it in l..h}

    pub fn sequential_digits(l: i32, h: i32) -> Vec<i32> {
        (2..10).flat_map(|w|(1..11-w).map(move |x| (x..x+w).fold(0,|r,t|r*10+t)))
               .filter(|&x|l<=x&&x<=h).collect()
    }