376. 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?

简单点就是求构成跌宕起伏、波动的数的最大数目。

只要前后波动变化就就加一，然后就没然后了，循环计算判断就好了。

设置一个flag表示前面的波动，0表示开始或开始时没有波动、-1表示向下波动、1表示向上波动，

class Solution {
public:
    int wiggleMaxLength(vector<int>& nums) {
        int len = nums.size();
        if(len < 1){
            return 0;
        }
        int flag =0;
        int count=1;
        for(int i=1; i<len; i++){
                if(nums[i] - nums[i-1] > 0){
                    if(flag == 0 || flag == -1){
                        count++;
                    }
                    flag = 1;
                }
                else if(nums[i] - nums[i-1] < 0){
                    if(flag == 0 || flag == 1){
                        count++;
                    }
                    flag = -1;
                }
        }
        return count;
    }
};