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 is a bucketing problem. You want buckets of 0s to pair with 75s, 1s to pair with 74s, etcetera. Bucketing is a Dictionary jobby. A Dictionary> gives you a result in O(n) amortized. If you only care about a bool result then a HashSet is good enough. You can't get better than O(n).

    static bool PairExists(int[] arr, int sum) {
        var set = new HashSet();
        foreach (int elem in arr) set.Add(elem);
        foreach (int elem in set)
            if (set.Contains(sum - elem)) return true;
        return false;
    }

If the array is likely to contain the pair then you could consider testing after the Add() call, still O(n).