March 30, 2026 · 4 min read

Two Sum with Hash Map: My Reliable O(n) Pattern

A practical walkthrough of how I use a hash map for Two Sum, including edge-case checks and why one-pass lookup is enough.

AlgorithmsData StructuresLeetCode

Source

Two Sum

Open original reference

Complexity

Time: O(n)

Space: O(n)

Problem in My Words

Given a list of numbers and a target value, find two different indices whose values add up to the target.

Approach

Scan once and store seen numbers in a hash map. For each number, compute its complement. If complement exists in the map, return indices.

Key Points

Use one pass to avoid nested loops.
Map stores value to index for quick lookup.
Check complement before inserting current element.

Implementation

function twoSum(nums: number[], target: number): [number, number] {
  const seen = new Map<number, number>();

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

  throw new Error("No valid pair found");
}