# 03.02.2025 [3105. Longest Strictly Increasing or Strictly Decreasing Subarray]

Longest strict monotonic subarray #easy

03.02.2025

3105. Longest Strictly Increasing or Strictly Decreasing Subarray easy blog post substack youtube

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Problem TLDR

Longest strict monotonic subarray #easy

Intuition

Don't forget we can use brute force when the problem size is small. Sometimes that code can be easy to write and check.

Approach

• the optimal solution is not that different from the brute force: drop the counter to 1

Complexity

• Time complexity: O(n), O(n^2) for the brute-force

• Space complexity: O(1)

Code

    fun longestMonotonicSubarray(nums: IntArray) = 
        nums.indices.maxOf { i -> 
            var a = i + 1; var b = a
            while (a < nums.size && nums[a - 1] > nums[a]) a++
            while (b < nums.size && nums[b - 1] < nums[b]) b++
            max(b - i, a - i) }

    pub fn longest_monotonic_subarray(nums: Vec<i32>) -> i32 {
        nums.chunk_by(|a, b| a > b).chain(nums.chunk_by(|a, b| a < b))
        .map(|c| c.len() as _).max().unwrap_or(1)
    }

    int longestMonotonicSubarray(vector<int>& n) {
        int a = 1, b = 1, r = 1;
        for (int i = 1; i < size(n); ++i)
            r = max({r, n[i] > n[i - 1] ? ++a : (a = 1),
                        n[i] < n[i - 1] ? ++b : (b = 1)});
        return r;
    }