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.lua

arr = {2, 7, 11, 4, 5}
target = 9
seen = {}
first = -1
second = -1
for i = 1, #arr do
	value = arr[i]
	need = target - value
	if seen[need] ~= nil then
		first = seen[need]
		second = i - 1
		break
	end
	seen[value] = i - 1
end
print("[" .. 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 Lua DSA version keeps the same data and final output as every other DSA book in this wave.