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.go

package main

import "fmt"

func main() {
	arr := []int{2, 7, 11, 4, 5}
	target := 9
	seen := map[int]int{}
	first := -1
	second := -1
	for i, value := range arr {
		need := target - value
		if j, ok := seen[need]; ok {
			first = j
			second = i
			break
		}
		seen[value] = i
	}
	fmt.Printf("[%d, %d]\n", 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 Go DSA version keeps the same data and final output as every other DSA book in this wave.