Two Sum

Write two_sum(nums, target) that returns [i, j] with i < j such that nums[i] + nums[j] == target, or None if no such pair exists.

• If several pairs work, any one is accepted.
• You may not use the same index twice (but two different indexes may hold equal values).
• 1 <= len(nums) <= 2 * 10^5; values are in [-10^9, 10^9] and target is in [-2 * 10^9, 2 * 10^9].
• Checking every pair is O(n²), too slow at this size. Aim for O(n).

two_sum([4, 11, -3, 8], 5)   # [2, 3]   (-3 + 8)
two_sum([6, 6], 12)          # [0, 1]
two_sum([5], 10)             # None