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

Swift infers arr as [Int] from let arr = [2, 7, 11, 4, 5]; the array and target bindings are immutable.
var seen: [Int: Int] = [:] is a mutable dictionary from value to prior index.
first and second are mutable Int sentinels initialized to -1 and are replaced only after a complement hit.
The loop uses for i in 0..<arr.count, reads let value = arr[i], and computes let need = target - value.
Dictionary lookup uses optional binding: if let j = seen[need]. On a miss, the optional is nil; on a hit, j is the stored prior index.
The code writes seen[value] = i only after checking the complement, so an element cannot pair with itself at the same index.
The trace records an initial miss for need=7 and map state {2: 0}, then a hit at i=1 with need=2, producing result [0, 1].
print("[\(first), \(second)]") formats the two sentinel/result values and prints [0, 1].

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.