Group by Key - C# 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 C# DSA implementation can be compared directly with the rest of the DSA track.

Basic Implementation

basic.cs

using System;
using System.Collections.Generic;
using System.Linq;

var pairs = new[] {("a", 1), ("b", 2), ("a", 3), ("c", 4), ("b", 5)};
var groups = new Dictionary<string, List<int>>();
var order = new List<string>();
foreach (var (key, value) in pairs) {
    if (!groups.ContainsKey(key)) {
        groups[key] = new List<int>();
        order.Add(key);
    }
    groups[key].Add(value);
}
var parts = order.Select(key => $"{key}: [{string.Join(", ", groups[key])}]");
Console.WriteLine("{" + string.Join(", ", parts) + "}");

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.