Arrays and Iteration

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

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.

Basic Implementation

basic.cs
Replay: real traced execution (multi-file project)
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}]");
	}
}

  1. arr ← [2, 7, 11, 4, 5], target ← 9, seen ← {}

    1using System;2using System.Collections.Generic;
    values this step[2, 7, 11, 4, 5]arr9target{}seen

  2. seen ← {2: 0}, hit ← no

    12int value = arr[i];13int need = target - value;14if (seen.ContainsKey(need)) {
    values this step{} → {2: 0}seennohit0i2arr[i]7need

  3. hit ← yes, result ← [0, 1]

    12int value = arr[i];13int need = target - value;14if (seen.ContainsKey(need)) {
    values this stepyeshit[0, 1]result1i7arr[i]2need{2: 0}seen

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.
  • Dictionary<int, int> stores the last seen index for each value; integer hashing gives average O(1) lookup, while collisions are handled inside the container. Assigning seen[value] = i intentionally overwrites duplicate values with the latest index.
  • The int[] input is a managed reference array, so arr[i] reads are bounds-checked by the CLR while arr.Length drives the scan.