Two Sum

Given a list and a target, return the indices of the two numbers summing to it — in one pass, not nested loops. Print the result for three cases.

[0, 1]
[1, 2]
[]

Keep a dict of value→index as you go. For each number, check whether target - number is already in it.

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

print(two_sum([2, 7, 11, 15], 9))
print(two_sum([3, 2, 4], 6))
print(two_sum([1, 2, 3], 99))

Run your code to check it…