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Summarized question: Return the indices of the two numbers in nums that add up to target.

Approach: one pass with a hash map. O(n) time, O(n) space.

For each number, check whether target minus it was already seen.

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