LeetCode-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?

Analysis:

Not hard, the longest wiggle subsequence is the sequence composed of all polar points. NOTE: be careful with corner cases: equal nums, all equal nums.

Solution:

public class Solution {
    public int getDirect(int curNum, int nextNum){
        return (curNum-nextNum)/Math.abs(curNum-nextNum);
    }
    public int wiggleMaxLength(int[] nums) {
        if (nums.length <=1 ) return nums.length;

        int res = 0;
        int curInd = 0;
        int nextInd = 0;
        while (nextInd < nums.length && nums[curInd] == nums[nextInd]) nextInd++;
        if (nextInd >= nums.length) return res+1;

        int preDirect = getDirect(nums[nextInd],nums[curInd]);
        while (nextInd < nums.length){
            int curDirect = getDirect(nums[curInd],nums[nextInd]);
            if (curDirect != 0 && curDirect != preDirect){
                res++;
                preDirect = curDirect;
            }
            curInd++;
            while (nextInd < nums.length && nums[curInd] == nums[nextInd]) nextInd++;
        }
        return res+1;
    }
}