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.py

arr = [2, 7, 11, 4, 5]
target = 9
seen = {}
result = None
for i, value in enumerate(arr):
    need = target - value
    if need in seen:
        result = [seen[need], i]
        break
    seen[value] = i
print(result)

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 Python DSA version keeps the same data and final output as every other DSA book in this wave.