Given an array of sorted values, return the number of unique values with O(n) complexity

countUniqueValues.js

/*
** Given an array of sorted values, return the number of unique values
**
** Method: Use 2 pointers (lastUniqueIndex and i) to mutate sorted array and
** return lastUniqueIndex + 1 for result
*/

function countUniqueValues(arr) {
  if (!arr.length) return 0
  let lastUniqueIndex = 0;

  for (let i = 1; i < arr.length; i++) {
    if (arr[lastUniqueIndex] !== arr[i]) {
      lastUniqueIndex++
      arr[lastUniqueIndex] = arr[i]
    }
  }

  return lastUniqueIndex + 1
}

countUniqueValues([1, 1, 2]) // 2
countUniqueValues([1, 2, 3, 3, 4]) // 4
countUniqueValues([]) // 0
countUniqueValues([-2, -1, -1, 0, 1]) // 4