Longest alternating subarray

Array Enumeration

35.1% acceptance

Feb 25, 2026

245

188

You are given a 0-indexed integer array nums. A subarray s of length m is called alternating if: m is greater than 1. s1 = s0 + 1. The 0-indexed subarray s looks like [s0, s1, s0, s1,...,s(m-1) % 2]. In other words, s1 - s0 = 1, s2 - s1 = -1, s3 - s2 = 1, s4 - s3 = -1, and so on up to s[m - 1] - s[m - 2] = (-1)m. Return the maximum length of all alternating subarrays present in nums or -1 if no such subarray exists. A subarray is a contiguous non-empty sequence of elements within an array.

Solution

Rust

Time O(n²)

Space O(1)

LeetCode

solution.rs

impl Solution {
  pub fn alternating_subarray(nums: Vec<i32>) -> i32 {
    let n = nums.len();
    let mut res = -1i32;
    for i in 0..n - 1 {
      if nums[i + 1] - nums[i] == 1 {
        let mut len = 2usize;
        let mut j = i + 1;
        while j + 1 < n {
          // expected diff at step len-1:
          // step 0: +1, step 1: -1, step 2: +1, ...
          let expected = if len % 2 == 0 { -1 } else { 1 };
          if nums[j + 1] - nums[j] == expected {
            len += 1;
            j += 1;
          } else {
            break;
          }
        }
        res = res.max(len as i32);
      }
    }
    res
  }
}