題目：
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.
Example 1:
Input: [1,7,4,9,2,5] Output: 6 Explanation: The entire sequence is a wiggle sequence.
Example 2:
Input: [1,17,5,10,13,15,10,5,16,8] Output: 7 Explanation: There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].
Example 3:
Input: [1,2,3,4,5,6,7,8,9] Output: 2
Follow up:
Can you do it in O(n) time?
代碼：
class Solution {
public:
int wiggleMaxLength(vector<int>& nums) {
int len = nums.size();
if (len == 0)return 0;
int res = 1, direct = -1;
for (int i = 1; i < len; i++) {
if (nums[i] > nums[i - 1]) {
if (direct!=1) {
res += 1;
direct = 1;
}
}
else if (nums[i] < nums[i - 1]) {
if (direct!=0) {
res += 1;
direct = 0;
}
}
}
return res;
}
};
理解本質，使用direct=-1;=0;=1表示不同的狀態根據不同的狀態進行求解。