题目描述:

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?

使用动态规划, 只需要保存当前节点与之前一个节点的信息.

class Solution {
public:
    int wiggleMaxLength(vector<int>& nums) {
        int len = nums.size();
        if(len <= 2) return len;
        int maxLen = 2, diff = nums[1] - nums[0];
        for(int i = 2; i < len; i++){
            int d = nums[i] - nums[i - 1];
            if(d && diff && ((d ^ diff) & 0x80000000)){
                maxLen++;
            }
            if(d != 0){
                diff = d;
            }
        }
        return maxLen;
    }
};