Two Sum in Python — return indices of two numbers that add to a target

Code example

from typing import Optional

def two_sum(nums: list[int], target: int) -> Optional[list[int]]:
    seen: dict[int, int] = {}  # value -> index
    for i, num in enumerate(nums):
        complement = target - num
        if complement in seen:
            return [seen[complement], i]
        seen[num] = i
    return None

# Tests
print(two_sum([2, 7, 11, 15], 9))  # [0, 1]
print(two_sum([3, 2, 4], 6))        # [1, 2]
print(two_sum([3, 3], 6))           # [0, 1]

# Sorted array variant (O(1) space with two pointers)
def two_sum_sorted(nums: list[int], target: int) -> list[int]:
    lo, hi = 0, len(nums) - 1
    while lo < hi:
        s = nums[lo] + nums[hi]
        if s == target:
            return [lo, hi]
        elif s < target:
            lo += 1
        else:
            hi -= 1
    return []