0376-wiggle-subsequence.js

/**
 * 376. Wiggle Subsequence
 * https://leetcode.com/problems/wiggle-subsequence/
 * Difficulty: 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.
 */

/**
 * @param {number[]} nums
 * @return {number}
 */
var wiggleMaxLength = function(nums) {
  let up = 1;
  let down = 1;

  for (let i = 1; i < nums.length; i++) {
    if (nums[i] > nums[i - 1]) {
      up = down + 1;
    } else if (nums[i] < nums[i - 1]) {
      down = up + 1;
    }
  }

  return Math.max(up, down);
};