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

AC代码，runtime 0ms.

 class Solution {
 public:
     int wiggleMaxLength(vector<int>& nums) {
         if(nums.empty())return ;
         int ret=,length=nums.size(),diff,bg=;
         for(;bg<length;bg++){
             diff=nums[bg]-nums[bg-];
             if(diff!=){
                 ret++;
                 break;
             }
             if(bg==length)return ret;
         }
         for(int i=bg+;i<length;i++){
             int tmpdiff=nums[i]-nums[i-];
             if(tmpdiff==)continue;
             if(tmpdiff*diff<){
                 diff=tmpdiff;
                 ret++;
             }
         }
         return ret;
     }
 };