LeetCode 376 Wiggle Subsequence (模拟)

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.

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

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

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

题目分析：容易发现题目就是要求数字序列单调性改变的次数

class Solution {
    public int wiggleMaxLength(int[] nums) {
        if (nums.length == 0) {
            return 0;
        }
        int ans = 1, i = 1;
        while (i < nums.length && nums[i] == nums[0]) {
           i++;
        }
        if (i == nums.length) {
            return 1;
        }
        ans++;
        boolean isIncr = nums[i - 1] < nums[i];
        for (; i < nums.length - 1; i++) {
            if (nums[i] < nums[i + 1] && !isIncr) {
                ans++;
                isIncr = true;
            } else if (nums[i] > nums[i + 1] && isIncr) {
                ans++;
                isIncr = false;
            }
        }
        return ans;
    }
}