Go Junior (0–2 yrs) Coding Frequent Go arrays maps Two Sum LeetCode

Two Sum in Go — return indices of two numbers that add up to a target

go-jun-005

Your answer

Answer as you would in a real interview — explain your thinking, not just the conclusion.

Model answer

Use a map[int]int to store value-to-index mappings as we iterate. For each element, compute complement = target - nums[i] and check if it exists in the map. If yes, return [stored index, i]. If no, store nums[i] -> i and continue. O(n) time, O(n) space. Maps in Go are hash maps, so lookups are amortised O(1).

Code example

package main

import "fmt"

func twoSum(nums []int, target int) []int {
    seen := make(map[int]int) // value -> index
    for i, n := range nums {
        complement := target - n
        if j, ok := seen[complement]; ok {
            return []int{j, i}
        }
        seen[n] = i
    }
    return nil // no solution
}

func main() {
    fmt.Println(twoSum([]int{2, 7, 11, 15}, 9)) // [0 1]
    fmt.Println(twoSum([]int{3, 2, 4}, 6))        // [1 2]
}

Follow-up

What would you change if the array were sorted and you needed O(1) space? (Two-pointer technique.)