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]

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.

思路: 假設在下標為 i 的位置已經知道了前面最長wiggle subsequence 的值， 那麽對於 i 這個位置wiggle subsequence 的值要麽和前面一樣，要麽比之前多1;

 1 class Solution {
 2 public:
 3     int wiggleMaxLength(vector<int>& nums) {
 4         if (nums.size() <= 1)
 5             return nums.size();
 6         int dp[nums.size()], diff = nums[1
 
] - nums[0]; dp[0] = 1;
 7         int flag = (diff > 0 ? -1 : 1);
 8         for (int i = 1; i != nums.size(); ++i){
 9             diff = nums[i] - nums[i-1];
10             if ((flag > 0 && diff > 0) || (flag < 0 && diff < 0) || !diff)
11                 dp[i] = dp[i-1];
12             else {
13                 dp[i] = dp[i-1] + 1;
14                 flag = diff;
15             }
16         }
17         return dp[nums.size() - 1];
18     }
19 };

[Leetcode]376. Wiggle Subsequence