2765. Longest Alternating Subarray 

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class Solution {
    public int alternatingSubarray(int[] nums) {
        int maxLength = -1;

        for (int i = 0; i < nums.length - 1; ) { // 枚举起点
            if (nums[i] + 1 == nums[i + 1]) {
                // 可作为交替子数组开头，尝试向右扩张
                int j = i + 2;
                int exceptedDiff = -1;
                while (j < nums.length && nums[j] == nums[j - 1] + exceptedDiff) {
                    exceptedDiff *= -1;
                    j++;
                }
                // now: [i, j)
                maxLength = Math.max(maxLength, j - i);
                i = j - 1; // 注意此处需要从 j - 1 从新开始尝试，如数组 [1, 2, 3, 2], 其中的最长交替子数组为 [2, 3, 2]
            } else {
                i++;
            }
        }

        return maxLength;
    }
}

References

2765. Longest Alternating Subarray