Two Sum – LeetCode

Given an array of integers, find two numbers such that they add up to a specific target number.

The function twoSum should return indices of the two numbers such that they add up to the target, where index1 must be less than index2. Please note that your returned answers (both index1 and index2) are not zero-based.

You may assume that each input would have exactly one solution.

Input: numbers={2, 7, 11, 15}, target=9
Output: index1=1, index2=2

Solution 1 – using HashMap
Analysis: O(n) runtime, O(n) space

Solution 2 – Brute force
Save the numbers in the set. Scan the numbers: pick one number #1, if the balance (target – #1) is in the set, then another number (#2) exists. Scan the rest numbers to find the index of #2.
Analysis: O(n2) runtime, O(n) space

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