Two Sum | DSA Journal

Question

Given an array of integers nums and an integer target, return the indices of the two numbers such that they add up to target. You may assume that each input would have exactly one solution, and you may not use the same element twice. You can return the answer in any order.

Example

Input: nums = [2, 7, 11, 15], target = 9

Output: [0, 1]

Explanation: nums[0] + nums[1] == 9

My Notes

Brute force is O(n²) — not efficient.
Use a hash map to track number → index.
For each element, check if target - num is in the map.
If yes, return the current index and the stored one.
This reduces the complexity to O(n).

func twoSum(nums []int, target int) []int {
  seen := make(map[int]int)
  for i, num := range nums {
    if j, ok := seen[target-num]; ok {
      return []int{j, i}
    }
    seen[num] = i
  }
  return nil
}

function twoSum(nums, target) {
  const seen = {};
  for (let i = 0; i < nums.length; i++) {
    const complement = target - nums[i];
    if (seen[complement] !== undefined) {
      return [seen[complement], i];
    }
    seen[nums[i]] = i;
  }
  return [];
}