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 0;
        int ret=1,length=nums.size(),diff,bg=1;
        for(;bg<length;bg++){
            diff=nums[bg]-nums[bg-1];
            if(diff!=0){
                ret++;
                break;
            }
            if(bg==length)return ret;
        }
        for(int i=bg+1;i<length;i++){
            int tmpdiff=nums[i]-nums[i-1];
            if(tmpdiff==0)continue;
            if(tmpdiff*diff<0){
                diff=tmpdiff;
                ret++;
            }
        }
        return ret;
    }
};