Two Sum

The problem. Given an array of integers nums and an integer target, return the indices of the two numbers that add up to target. Exactly one answer exists, and you may not use the same element twice.

The array [2, 7, 11, 15] with target 9. The cells at index 0 and 1 are highlighted because 2 + 7 = 9, so the answer is [0, 1].

Example

Input:  nums = [2, 7, 11, 15], target = 9
Output: [0, 1]      because nums[0] + nums[1] = 2 + 7 = 9

Input:  nums = [3, 2, 4], target = 6
Output: [1, 2]      3 + 3 would be 6, but that uses index 0 twice

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Still to come

1. Brute force: try every pair
2. Optimal: sort, then walk two pointers inward
3. Best solution: one pass with a hash map
Comparing the three approaches
Edge cases to test

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Two Sum

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