Find if pair of elements with given sum exists in large integer array

Question

I\'m having a integer array of 10 Million elements, how to write a function in C# which returns True if the array has a pair which sums up to 75.

My code is:

Answer 1

If you can assume the numbers are positive, you can do this:

1. Create a boolean array of 75 elements.
2. Walk the list of 10,000,000 items. When you see one that's 75 or less:
   • check to see boolean arry has an entry at element 75 - that number. If it does, you found a match.
   • otherwise, set the nth entry of the boolean array.

Example C#:

        var bits = new bool[75];
        foreach (var n in intArray)
        {
            if (n <= 75)
            {
                var diff = 75 - n;
                if (bits[diff - 1])
                {
                    MessageBox.Show(string.Format("Found pair: {0} and {1}", n, diff));
                    break;
                }
                bits[n - 1] = true;
            }
        }

But if the numbers in the array can be any valid integer, including negative numbers, you can do something like this:

        var set = new HashSet();
        foreach (var n in intArray)
        {
            if (n <= 75)
            {
                var diff = 75 - n;

                if (set.Contains(diff))
                {
                    MessageBox.Show(string.Format("Found pair: {0} and {1}", n, diff));
                    break;
                }
                set.Add(n);
            }
        }