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:

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       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
      
      
      
      

       
       
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Follow up:
Can you do it in O(n) time?

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       class Solution
       
       
    public int wiggleMaxLength(int[] nums) {
       
       
        if(nums.length <= 1) return nums.length;  
       
       
        int count = 1;  
       
       
        Boolean prevInc = null;  
       
       
        for(int i = 1; i < nums.length; i++) {  
       
       
            if(nums[i] > nums[i - 1] && (prevInc == null || !prevInc)) {  
       
       
                prevInc = true;  
       
       
                count++;  
       
       
            } else if(nums[i] < nums[i - 1] && (prevInc == null || prevInc)) {  
       
       
                prevInc = false;  
       
       
                count++;  
       
       
            }  
       
       
        }  
       
       
        return count;  
       
       
    }
       
       
}
      
      
      
      

       
       
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