Wiggle Subsequence

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?

 

Runtime: 0ms. 

class Solution {
public:
    int wiggleMaxLength(vector<int>& nums) {
        int positive = 1, negative = 1, n = nums.size();
        for(int i = 1; i < nums.size(); i++) {
            if(nums[i] > nums[i - 1]) positive = negative + 1;
            else if(nums[i] < nums[i - 1]) negative = positive + 1;
        }
        return min(n, max(positive, negative));
    }
};