[LeetCode] (medium) 376. Wiggle Subsequence

https://leetcode.com/problems/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.

Example 1:

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

Example 2:

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

Example 3:

Input: [1,2,3,4,5,6,7,8,9]
Output: 2

Follow up:
Can you do it in O(n) time?

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想象成折线图就很容易了 

class Solution {
public:
    int wiggleMaxLength(vector<int>& nums) {
                
        if(nums.size() == 0) return nums.size();
        
        int st = 1;
        while(st < nums.size() && nums[st] == nums[st-1]) ++st;
        if(st == nums.size()) return 1;
        
        bool neg;
        int result = 2;
        neg = ((nums[st] - nums[st-1]) < 0);
        for(int i = st+1; i < nums.size(); ++i){
            if(nums[i] == nums[i-1]) continue;
            if(neg == ((nums[i] - nums[i-1]) < 0)) continue;
            else{
                neg = !neg;
                ++result;
            }
        }
        
        return result;
    }
};