978. Longest Turbulent Subarray

A subarray A[i], A[i+1], ..., A[j] of A is said to be turbulent if and only if:

• For i <= k < j, A[k] > A[k+1] when k is odd, and A[k] < A[k+1] when k is even;
• OR, for i <= k < j, A[k] > A[k+1] when k is even, and A[k] < A[k+1] when k is odd.

That is, the subarray is turbulent if the comparison sign flips between each adjacent pair of elements in the subarray.

Return the length of a maximum size turbulent subarray of A.

Example 1:

Input: [9,4,2,10,7,8,8,1,9]
Output: 5
Explanation: (A[1] > A[2] < A[3] > A[4] < A[5])

Example 2:

Input: [4,8,12,16]
Output: 2

Example 3:

Input: [100]
Output: 1

Note:

1. 1 <= A.length <= 40000
2. 0 <= A[i] <= 10^9

Approach #1: Math. [Java]

class Solution {
    public int maxTurbulenceSize(int[] A) {
        int len = A.length;
        int inc = 1, dec = 1, result = 1;
        for (int i = 1; i < len; ++i) {
            if (A[i] < A[i-1]) {
                dec = inc + 1;
                inc = 1;
            } else if (A[i] > A[i-1]) {
                inc = dec + 1;
                dec = 1;
            } else {
                inc = 1;
                dec = 1;
            }
            
            result = Math.max(result, Math.max(inc, dec));
        }
        
        return result;
    }
}

　　

Analysis:

inc: denote the length of subarray with two increase elements;

dec: denote the length of subarray with two decrease elements;

Reference:

https://leetcode.com/problems/longest-turbulent-subarray/discuss/221935/Java-O(N)-time-O(1)-space