Two-Sum with Hash Lookup

Walk the array once, storing seen values in a lookup table. When the complement is already present, the result indices are known.

Algorithm

Basic Implementation

basic.scala

object Main {
	def main(args: Array[String]): Unit = {
		val arr = Array(2, 7, 11, 4, 5)
		val target = 9
		val seen = scala.collection.mutable.Map[Int, Int]()
		var first = -1
		var second = -1
		var done = false
		for (i <- arr.indices if !done) {
			val value = arr(i)
			val need = target - value
			if (seen.contains(need)) {
				first = seen(need)
				second = i
				done = true
			} else {
				seen(value) = i
			}
		}
		println(s"[$first, $second]")
	}
}

Complexity

Time: O(n) average
Space: O(n)

Implementation notes

val arr = Array(2, 7, 11, 4, 5) and val target = 9 are immutable Scala bindings for the checked input; the array elements are read with arr(i).
val seen = scala.collection.mutable.Map[Int, Int]() stores value-to-index pairs and is mutated in place.
var first = -1 and var second = -1 are sentinel result slots until a pair is found; var done = false stops later loop iterations.
The loop for (i <- arr.indices if !done) uses the array's valid index range and a guard so the scan stops after the first hit.
Each step binds val value = arr(i) and val need = target - value.
Lookup uses seen.contains(need) followed by seen(need) on a hit; this source does not use an Option-returning lookup.
On a miss, seen(value) = i records the current value and index. The trace shows i = 0 needing 7, missing, and writing {2: 0}.
At i = 1, value 7 needs 2, hits the map, sets result [0, 1], and println(s"[$first, $second]") prints [0, 1].

execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.

cross-language comparison This Scala DSA version keeps the same data and final output as every other DSA book in this wave.