Arrays and Iteration

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.js 
const arr = [2, 7, 11, 4, 5];
const target = 9;
const seen = new Map();
let result = null;
for (let i = 0; i < arr.length; i++) {
  const need = target - arr[i];
  if (seen.has(need)) {
    result = [seen.get(need), i];
    break;
  }
  seen.set(arr[i], i);
}
console.log(`[${result[0]}, ${result[1]}]`);

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