1. Two Sum

Use a hash map to store each number's index. For each number, check if the complement (target - num) already exists in the map.

Walk-through

1. Create an empty hash map seen.
2. Iterate through the array with index i and value num.
3. Compute complement = target - num.
4. If complement is in seen, return [seen[complement], i].
5. Otherwise, store num → i in seen.

Complexity

• Time: O(n) — single pass through the array.
• Space: O(n) — hash map stores at most n entries.

import java.util.HashMap;
import java.util.Map;

class Solution {
    public int[] twoSum(int[] nums, int target) {
        Map<Integer, Integer> seen = new HashMap<>();
        for (int i = 0; i < nums.length; i++) {
            int complement = target - nums[i];
            if (seen.containsKey(complement)) {
                return new int[]{seen.get(complement), i};
            }
            seen.put(nums[i], i);
        }
        return new int[]{};
    }
}