leetcode 376. Wiggle Subsequence
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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: 6The entire sequence is a wiggle sequence.Input: [1,17,5,10,13,15,10,5,16,8]Output: 7There 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: 2Follow up:Can you do it in O(n) time?
这种有状态的题已经出现好多次了,大多数都可以设定状态转移的过程,每种状态设一个数组。
这里就是出于up 和 down 和equal的情况
解法如下:
public class Solution { public int wiggleMaxLength(int[] nums) { if(nums==null||nums.length==0) return 0; int len = nums.length; int[] up = new int[len]; int[] down = new int[len]; up[0] = 1; down[0] = 1; for(int i=1; i < len; 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 Math.max(up[len-1], down[len-1]); }}
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