TwoSum (C#) : Algorithm

Write a function that, given a list and a target sum, returns zero-based indices of any two distinct elements whose sum is equal to the target sum. If there are no such elements, the function should return null.

For example, FindTwoSum(new List<int>() { 1, 3, 5, 7, 9 }, 12) should return any of the following tuples of indices:

• 1, 4 (3 + 9 = 12)
• 2, 3 (5 + 7 = 12)
• 3, 2 (7 + 5 = 12)
• 4, 1 (9 + 3 = 12)

E.g

Code Snippet

1. using System;
2. using System.Collections.Generic;
3.  
4. class TwoSum
5. {
6.     public static Tuple<int, int> FindTwoSum(IList<int> list, int sum)
7.     {
8.         throw new NotImplementedException("Waiting to be implemented.");
9.     }
10.  
11.     public static void Main(string[] args)
12.     {
13.         Tuple<int, int> indices = FindTwoSum(new List<int>() { 1, 3, 5, 7, 9 }, 12);
14.         Console.WriteLine(indices.Item1 + " " + indices.Item2);
15.     }
16. }

Solved Code Snippet

1. using System;
2. using System.Linq;
3. using System.Collections.Generic;
4.  
5. class TwoSum
6. {
7.     public static Tuple<int, int> FindTwoSum(IList<int> list, int sum)
8.     {
9.         var result = from n1 in list
10.                      from n2 in list
11.                      where n1 + n2 == sum
12.                      select new Tuple<int, int>(list.IndexOf(n1), list.IndexOf(n2));
13.         return result.FirstOrDefault();
14.     }
15.  
16.     public static void Main(string[] args)
17.     {
18.         Tuple<int, int> indices = FindTwoSum(new List<int>() { 1, 3, 5, 7, 9 }, 12);
19.         Console.WriteLine(indices.Item1 + " " + indices.Item2);
20.     }
21. }