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

This will work if the array is sorted.

public bool ContainsPair(int[] array) 
{
    int i = 0;
    int j = array.Length - 1;
    while(i < j)
    {
        if (array[i] + array[j] == 75) 
            return true;
        else if (array[i] + array[j] <  75) 
            i++;
        else if (array[i] + array[j] >  75) 
            j--;
    }
    return false;
}

You use two pointers and walk towards the middle of the array. Pointer i starts at the beginning of the array, while j starts at the end. If you find two numbers that sum up to 75, you return true. If the sum is less than 75, then you move pointer i one step towards the middle and check again. If the sum is more than 75, you move pointer j one step towards the middle and check again.

If the two pointers meet, then you return false, because no pair was found.

This is O(n), not including sorting the array.