Print all unique pairs of numbers from a list that sum to 20.

To find all unique pairs that sum to 20, iterate through the list while keeping a hash set of numbers seen so far. For each number x, compute its complement y = 20 - x. If y is already in the set, output the pair (min(x,y), max(x,y)) to avoid ordering duplicates. Add x to the set after checking. This ensures each pair is reported once. Complexity is O(n) time and O(n) space. Example in Python:

nums = [5, 15, 10, 10, 5, 20, 0]
seen = set()
result = set()
for x in nums:
    y = 20 - x
    if y in seen:
        result.add(tuple(sorted((x, y))))
    seen.add(x)
print(result)  # {(5, 15), (10, 10), (20, 0)}

If you need sorted output, convert the set to a list and sort it.