【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.
【思路】
DP/贪心都可以，贪心更简单。
核心比较A[i+2]-A[i+1]和A[i+1]-A[i]是否异号，是则cnt++，否则i++循环。
两个指针和一个循环，特殊情况只有两个元素的数组单独讨论。
才发现可以插入代码！！好看多了……！
【代码】
class Solution {
public int wiggleMaxLength(int[] nums) {
if (nums.length<2)
return nums.length;
int p1=nums[1]-nums[0];
int cnt=p1!=0?2:1;
for(int i=2;i<nums.length;i++){
int p2=nums[i]-nums[i-1];
if((p1<=0&&p2>0)||(p1>=0&&p2<0)){
p1=p2;
cnt++;
}
}
return cnt;
}
}
【举例】
Example 1:
Input: [1,7,4,9,2,5]
Output: 6
Explanation: The entire sequence is a wiggle sequence.
Example 2:
Input: [1,17,5,10,13,15,10,5,16,8]
Output: 7
Explanation: There are several subsequences that achieve this length. One is [1,17,10,13,10,16,8].
Example 3:
Input: [1,2,3,4,5,6,7,8,9]
Output: 2