[Leetcode 376]摇摆序列 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.

【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