How to implement an algorithm that finds the first element from the first array that is missing in the second array?

To find the first element from the first array that is missing in the second, you can use a Set for quick lookup. First, create a Set from the second array, then iterate through the first array and return the first element not in this Set.

Example in Kotlin for Android:

fun findFirstMissingElement(arr1: List<Int>, arr2: List<Int>): Int? {
    val set2 = arr2.toSet()
    for (element in arr1) {
        if (element !in set2) {
            return element
        }
    }
    return null // if all elements of arr1 are in arr2
}

// Usage:
val firstArray = listOf(1, 2, 3, 4)
val secondArray = listOf(2, 3, 5)
val result = findFirstMissingElement(firstArray, secondArray) // returns 1

This approach is efficient in terms of time, as lookup in a Set is O(1), and the overall algorithm is O(n), where n is the length of the first array.