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

execution replay The checked-in replay follows the language-neutral state table for `array-two-sum-hash`.
cross-language comparison This Java DSA version keeps the same data and final output as every other DSA book in this wave.

Basic Implementation

Basic.java
Replay: real traced execution (multi-file project)
import java.util.*;

public class Basic {
    public static void main(String[] args) {
        int[] arr = {2, 7, 11, 4, 5};
        int target = 9;
        Map<Integer, Integer> seen = new HashMap<>();
        int first = -1;
        int second = -1;
        for (int i = 0; i < arr.length; i++) {
            int need = target - arr[i];
            if (seen.containsKey(need)) {
                first = seen.get(need);
                second = i;
                break;
            }
            seen.put(arr[i], i);
        }
        System.out.println("[" + first + ", " + second + "]");
    }
}

  1. arr ← [2, 7, 11, 4, 5], target ← 9, seen ← {}

    1import java.util.*;
    values this step[2, 7, 11, 4, 5]arr9target{}seen

  2. seen ← {2: 0}, hit ← no

    10for (int i = 0; i < arr.length; i++) {11    int need = target - arr[i];12    if (seen.containsKey(need)) {
    values this step{} → {2: 0}seennohit0i2arr[i]7need

  3. hit ← yes, result ← [0, 1]

    10for (int i = 0; i < arr.length; i++) {11    int need = target - arr[i];12    if (seen.containsKey(need)) {
    values this stepyeshit[0, 1]result1i7arr[i]2need{2: 0}seen

Complexity

  • Time: O(n) average
  • Space: O(n)

Implementation notes

  • Java keeps the input in a primitive int[], while seen is declared as Map<Integer, Integer> and instantiated with new HashMap<>(). Each key and stored index is boxed from int to Integer for the map.
  • The loop computes need = target - arr[i], calls seen.containsKey(need), and only then reads seen.get(need) on a hit. On a miss, seen.put(arr[i], i) records the current value as the key and the current index as the value.
  • This trace stops after the first hit, so only {2: 0} is inserted before arr[1] == 7 finds complement 2 and produces result [0, 1].
  • HashMap gives average O(1) lookup/update, with normal collision and resize behavior hidden below the replay. The array itself is not mutated; allocation is the map storage/nodes plus autoboxed Integer values managed by the JVM; small boxed values may come from the Integer cache.