def two_sum(nums, target):
seen = {}
for i, x in enumerate(nums):
if target - x in seen:
return [seen[target - x], i]
seen[x] = i
return []Variables
n- length of the input array nums
Explanation
Single linear pass with a hash lookup at each step - **O(n)** time. The map stores at most `n` entries, giving **O(n)** auxiliary space. The hash map turns the inner search from O(n) into O(1).