g0301_0400.s0376_wiggle_subsequence.Solution Maven / Gradle / Ivy

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package g0301_0400.s0376_wiggle_subsequence;

// #Medium #Array #Dynamic_Programming #Greedy #Dynamic_Programming_I_Day_18
// #2022_07_12_Time_0_ms_(100.00%)_Space_42.4_MB_(6.74%)

/**
 * 376 - Wiggle Subsequence\.
 *
 * Medium
 *
 * A **wiggle sequence** is a sequence where 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 one element and a sequence with two non-equal elements are trivially wiggle sequences.
 *
 * *   For example, `[1, 7, 4, 9, 2, 5]` is a **wiggle sequence** because the differences `(6, -3, 5, -7, 3)` alternate between positive and negative.
 * *   In contrast, `[1, 4, 7, 2, 5]` and `[1, 7, 4, 5, 5]` are not wiggle sequences. The first is not because its first two differences are positive, and the second is not because its last difference is zero.
 *
 * A **subsequence** is obtained by deleting some elements (possibly zero) from the original sequence, leaving the remaining elements in their original order.
 *
 * Given an integer array `nums`, return _the length of the longest **wiggle subsequence** of_ `nums`.
 *
 * **Example 1:**
 *
 * **Input:** nums = [1,7,4,9,2,5]
 *
 * **Output:** 6
 *
 * **Explanation:** The entire sequence is a wiggle sequence with differences (6, -3, 5, -7, 3).
 *
 * **Example 2:**
 *
 * **Input:** nums = [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] with differences (16, -7, 3, -3, 6, -8).
 *
 * **Example 3:**
 *
 * **Input:** nums = [1,2,3,4,5,6,7,8,9]
 *
 * **Output:** 2
 *
 * **Constraints:**
 *
 * *   `1 <= nums.length <= 1000`
 * *   `0 <= nums[i] <= 1000`
 *
 * **Follow up:** Could you solve this in `O(n)` time?
**/
public class Solution {
    public int wiggleMaxLength(int[] nums) {
        int lt = 1;
        int gt = 1;
        for (int i = 1; i < nums.length; i++) {
            if (nums[i - 1] < nums[i]) {
                lt = gt + 1;
            } else if (nums[i - 1] > nums[i]) {
                gt = lt + 1;
            }
        }
        return Math.max(lt, gt);
    }
}