Zig Zag

isZigZag.js

const isZigZag = str => {
  const nums = str.split(" ").map(Number)
  return nums.every(
    (num, i) =>
      num > Math.max(nums[i - 1] || -Infinity, nums[i + 1] || -Infinity) ||
      num < Math.min(nums[i - 1] || +Infinity, nums[i + 1] || +Infinity)
  )
}

isZigZag("4 2 3 1 5 3")
// -> true
isZigZag("7 3 5 5 2")
// -> false
isZigZag("3 8 6 4 5")
// -> false
isZigZag("1")
// -> true

longestZigZag.js

const longestZigZag = str => {
  const signs = str
    .split(" ")
    .map(Number)
    .map((n, i, arr) => Math.sign(n - arr[i - 1]))
  const lens = signs.reduce(
    (acc, n, i) =>
      n !== signs[i - 1] && n !== 0
        ? (([x, ...xs]) => [x + 1, ...xs])(acc)
        : [1, ...acc],
    [0]
  )
  return Math.max(...lens)
}