Problem - Two Sum

Given an array of integers $nums$ and a target integer $target$, find two indices $i$ and $j$ such that $nums[i] + nums[j] = target$.

For example, if $nums = [2, 7, 11, 15]$ and $target = 9$, the answer is indices $[0, 1]$ because $2 + 7 = 9$.

This is a classic problem that demonstrates algebraic thinking. Before reading on, think: what are you looking for? You need two numbers that add to the target.