Group by Key - Swift DSA

Build buckets keyed by a shared field, preserving the first-seen key order.

Algorithm

Canonical pairs (a,1), (b,2), (a,3), (c,4), (b,5) print {a: [1, 3], b: [2, 5], c: [4]}. The replay uses the same input in every language, so this Swift DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.swift

let pairs = [("a", 1), ("b", 2), ("a", 3), ("c", 4), ("b", 5)]
var groups: [String: [Int]] = [:]
var order: [String] = []
for (key, value) in pairs {
    if groups[key] == nil {
        groups[key] = []
        order.append(key)
    }
    groups[key]!.append(value)
}
let parts = order.map { key in "\(key): [\(groups[key]!.map(String.init).joined(separator: ", "))]" }
print("{\(parts.joined(separator: ", "))}")

Complexity

Time: O(n) average
Space: O(k + n) for buckets and values

Implementation notes

Keep output formatting deterministic. Do not rely on unordered hash-map printing when the lesson needs cross-language comparison.
The trace highlights the hash table state after each write.

bucket map Each key owns a list. A new key creates a bucket; a repeated key appends to the existing bucket.