Define an array of integers and write a function to determine if any two elements sum to a target value of 20.

A common way to solve the "two sum" problem is to use a hash set to keep track of the numbers we have seen so far. Iterate through the array once. For each element x, compute the complement target - x. If the complement is already in the set, we have found a pair that sums to the target and can return true (or the indices). If not, add x to the set and continue. This approach runs in O(n) time and uses O(n) additional space. In Python:

def has_pair_with_sum(arr, target):
    seen = set()
    for num in arr:
        if target - num in seen:
            return True
        seen.add(num)
    return False

If you need the indices, you can store a dictionary mapping numbers to their indices. This solution is efficient for large arrays and is a typical interview question for junior to mid-level developers.