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.cs

using System;
using System.Collections.Generic;

class Program {
	static void Main() {
		int[] arr = new int[] { 2, 7, 11, 4, 5 };
		int target = 9;
		var seen = new Dictionary<int, int>();
		int first = -1;
		int second = -1;
		for (int i = 0; i < arr.Length; i++) {
			int value = arr[i];
			int need = target - value;
			if (seen.ContainsKey(need)) {
				first = seen[need];
				second = i;
				break;
			}
			seen[value] = i;
		}
		Console.WriteLine($"[{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 C# DSA version keeps the same data and final output as every other DSA book in this wave.