Two-Sum with Hash Lookup

Walk the array once, storing seen values in a lookup table. When the complement is already present, the result indices are known.

Algorithm

Basic Implementation

basic.rb

arr = [2, 7, 11, 4, 5]
target = 9
seen = {}
first = -1
second = -1
i = 0
while i < arr.length
	value = arr[i]
	need = target - value
	if seen.key?(need)
		first = seen[need]
		second = i
		break
	end
	seen[value] = i
	i += 1
end
puts "[#{first}, #{second}]"

Complexity

Time: O(n) average
Space: O(n)

Implementation notes

Keep the explicit control flow. Library shortcuts would hide the state changes this lesson is meant to replay.
The final output is intentionally small and deterministic for cross-language comparison.

execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.

cross-language comparison This Ruby DSA version keeps the same data and final output as every other DSA book in this wave.