【LeetCode】 376. Wiggle Subsequence

最新推荐文章于 2018-11-16 17:30:00 发布

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本文介绍了一种高效算法，用于找出整数序列中最长的摆动子序列。摆动子序列是指序列中相邻元素之间的差值严格交替正负的子序列。文章提供了一个示例代码实现，并探讨了如何在O(n)时间内解决问题。

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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: 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

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

public class Solution {
    public int wiggleMaxLength(int[] nums) {
        int num1 = 1, num2 = 1;
        for (int i = 1; i < nums.length; i++) {
            if (nums[i] > nums[i - 1]) {
                num1 = num2 + 1;
            } else if (nums[i] < nums[i - 1]){
                num2 = num1 + 1;
            }
        }
        return Math.min(nums.length, Math.max(num1, num2));
    }
}