376.go

package p376

/**
A sequence of numbers is called a wiggle sequence if the differences
between successive numbers strictly alternate between positive and negative.
The first difference (if one exists) may be either positive or negative.
A sequence with fewer than two elements is trivially a wiggle sequence.

For example, [1,7,4,9,2,5] is a wiggle sequence because the differences
(6,-3,5,-7,3) are alternately positive and negative.
In contrast, [1,4,7,2,5] and [1,7,4,5,5] are not wiggle sequences,
the first because its first two differences are positive and
the second because its last difference is zero.

Given a sequence of integers, return the length of the longest subsequence that is a wiggle sequence.
A subsequence is obtained by deleting some number of elements (eventually, also zero)
from the original sequence, leaving the remaining elements in their original order.

Examples:
Input: [1,7,4,9,2,5]
Output: 6
The entire sequence is a wiggle sequence.

Input: [1,17,5,10,13,15,10,5,16,8]
Output: 7
There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].

Input: [1,2,3,4,5,6,7,8,9]
Output: 2
Follow up:
Can you do it in O(n) time?


*/

func sig(x int) int {
	if x < 0 {
		return -1
	} else if x > 0 {
		return 1
	}
	return 0
}

func wiggleMaxLength(nums []int) int {
	if nums == nil || len(nums) == 0 {
		return 0
	}
	pSig := 0
	cnt := 1
	for i := 1; i < len(nums); i++ {
		flag := sig(nums[i] - nums[i-1])
		if flag == 0 || flag == pSig {
			continue
		} else {
			pSig = flag
			cnt++
		}
	}
	return cnt
}