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.swift

let arr = [2, 7, 11, 4, 5]
let target = 9
var seen: [Int: Int] = [:]
var first = -1
var second = -1
for i in 0..<arr.count {
	let value = arr[i]
	let need = target - value
	if let j = seen[need] {
		first = j
		second = i
		break
	}
	seen[value] = i
}
print("[\(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 Swift DSA version keeps the same data and final output as every other DSA book in this wave.