Find two numbers in an unsorted list that add up to a target and return their indices.
Find the first character in a string that appears only once.
Group words that contain the exact same letters, like 'eat' and 'tea'.
Given ice cream prices and a budget, find two flavors that cost exactly the budget.
Count how many unique pairs of numbers in an array differ by exactly k.
Count how many contiguous subarrays add up to exactly k.