Arrays and Iteration

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.kt 
fun main() {
	val arr = intArrayOf(2, 7, 11, 4, 5)
	val target = 9
	val seen = mutableMapOf<Int, Int>()
	var first = -1
	var second = -1
	for (i in arr.indices) {
		val value = arr[i]
		val need = target - value
		if (seen.containsKey(need)) {
			first = seen[need]!!
			second = i
			break
		}
		seen[value] = i
	}
	println("[$first, $second]")
}

Complexity

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

Implementation notes

  • Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
  • The final output is intentionally small and deterministic for cross-language comparison.
execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.
cross-language comparison This Kotlin DSA version keeps the same data and final output as every other DSA book in this wave.