Problem 37: Two Sum

def two_sum(nums, target): """ Return the indices of the two numbers in `nums` that add up to `target`. The function assumes that exactly one such pair exists and that the same element cannot be used twice. It runs in O(n) time using a hash map. """ # Map from number value to its index (the first occurrence we have seen) seen = {} for i, num in enumerate(nums): complement = target - num if complement in seen: # Found the pair: return the earlier index and the current one return [seen[complement], i] # Store the current number only if we haven't seen it before. # This preserves the first occurrence, which is enough because the # problem guarantees a unique solution. if num not in seen: seen[num] = i # If the input respects the problem constraints, we should never reach here. raise ValueError("No two sum solution found")