LeetcodeWiggle Subsequence

Posted 2023-02-10 wuezs

题目链接：https://leetcode.com/problems/wiggle-subsequence/

题目：
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.

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?

思路：
贪心。。easy，要注意相等情况，当相等时，要用一个标记记录前面是递增的，还是递减的。

算法：

    public int wiggleMaxLength(int[] nums) 
        Boolean flag = null;//true为递增
        if (nums.length <= 1)
            return nums.length;
        int count = 1;

        for (int i = 1; i < nums.length; i++) 
            if(nums[i]>nums[i-1])//递增
                if(flag==null)//前面全相等
                    count++;
                    flag = true;
                else//前面位递减时
                    if(flag==false)
                        count++;
                        flag = true;
                    
                
            else if(nums[i]<nums[i-1])//与上同理
                if(flag==null)
                    count++;
                    flag = false;
                else
                    if(flag==true)
                        count++;
                        flag = false;
                    
                
            
        
        return count;