Write a function that, when passed a list and a target sum, returns, **efficiently** with respect to time used, two distinct** **zero-based indices of any two of the numbers, whose sum is equal to the target sum. If there are no two numbers, the function should return *null*.

For example, *FindTwoSum(new List<int>() { 3, 1, 5, 7, 5, 9 }, 10)* should return a *Tuple<int, int>* containing any of the following pairs of indices:

- 0 and 3 (or 3 and 0) as 3 + 7 = 10
- 1 and 5 (or 5 and 1) as 1 + 9 = 10
- 2 and 4 (or 4 and 2) as 5 + 5 = 10

- Example case: System.NotImplementedException at TwoSum.FindTwoSum(IList`1 list, Int32 sum) in TwoSum.cs:line 8
- Distinct numbers with and without solutions: System.NotImplementedException at TwoSum.FindTwoSum(IList`1 list, Int32 sum) in TwoSum.cs:line 8
- Duplicate numbers with and without solutions: System.NotImplementedException at TwoSum.FindTwoSum(IList`1 list, Int32 sum) in TwoSum.cs:line 8
- Performance test with a large list of numbers: System.NotImplementedException at TwoSum.FindTwoSum(IList`1 list, Int32 sum) in TwoSum.cs:line 8

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