We use a hash map to store the values we've seen so far and their indices. For each number x, we check if target - x is already in the map.

1. Initialize seen = {}.
2. Iterate through nums with index i:
   • complement = target - nums[i].
   • If complement is in seen, return [seen[complement], i].
   • Otherwise, seen[nums[i]] = i.

• Time Complexity: O(N).
• Space Complexity: O(N).

def two_sum(nums, target):
    seen = {}
    for i, num in enumerate(nums):
        complement = target - num
        if complement in seen:
            return [seen[complement], i]
        seen[num] = i