leetcode 376 - Wiggle Subsequence

https://leetcode.com/problems/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.

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?

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//O(N^2)
class Solution {
public:
int wiggleMaxLength(vector<int>& nums) {
int size = nums.size();

if (size == 0) return 0;

vector<int> up(size), down(size);

up[0] = 1;
down[0] = 1;

int max_len = 1;

for (int i = 1; i < size; i++) {
up[i] = down[i] = 1;
for (int j = 0; j < i; j++) {
if (nums[j] < nums[i])
up[i] = max(up[i], down[j]+1);

if (nums[j] > nums[i])
down[i] = max(down[i], up[j]+1);
}

max_len = max(max_len, max(up[i], down[i]));
}

return max_len;
}
};