Hash Tables

Group by Key

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 TypeScript DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.ts 
const pairs: Array<[string, number]> = [["a", 1], ["b", 2], ["a", 3], ["c", 4], ["b", 5]];
const groups = new Map<string, number[]>();
for (const [key, value] of pairs) {
  if (!groups.has(key)) {
    groups.set(key, []);
  }
  groups.get(key)!.push(value);
}
const parts: string[] = [];
for (const [key, values] of groups) {
  parts.push(`${key}: [${values.join(", ")}]`);
}
console.log(`{${parts.join(", ")}}`);

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.