Week13 - 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

Follow up:

Can you do it in O(n) time?

Credits:

Special thanks to @agave and @StefanPochmann for adding this problem and creating all test cases.

my solution:

class Solution {

public:

    int wiggleMaxLength(vector<int>& nums) {

        int size = nums.size();

        if (size == 0) {

            return 0;

        }

        vector<int> up(size, 0);

        vector<int> down(size, 0);

        up[0] = 1;

        down[0] = 1;

        for(int i=1; i<size; ++i){

            if (nums[i] > nums[i-1]) {

                up[i] = down[i-1] + 1;

                down[i] = down[i-1];

            }

            else if (nums[i] < nums[i-1]) {

                down[i] = up[i-1] + 1;

                up[i] = up[i-1];

            }

            else {

                up[i] = up[i-1];

                down[i] = down[i-1];

            }

        }

        return max(up[size-1], down[size-1]);

    }

};