Hash maps and sets · routine

Two sum

Write two_sum(nums, target) returning [i, j] with i < j and nums[i] + nums[j] == target, choosing the smallest j that works. Return [] if there is no such pair.

For each number, the partner you need is target - number. A dictionary from value to index tells you in one step whether you have already passed it.

right answers
arguments left as they should be
fast enough at scale

def two_sum(nums, target):
    for i in range(len(nums)):
        for j in range(i + 1, len(nums)):
            if nums[i] + nums[j] == target:
                return [i, j]
    return []

Run adds print(two_sum([2, 7, 11, 15], 9)) after your code, to try it.

Stuck on the idea rather than the code? The Hash maps and sets lesson walks through it.