Given unsorted array a and target t, return indices of the pair that sums to t.
Find the index of the first character that appears only once.
Group a list of words by anagram.
Given prices and a budget, find two items that sum to exactly the budget.
Count unique pairs where |a-b| = k.
Determine if a number is 'happy': repeatedly sum the squares of its digits; if it reaches 1 it's happy.
Deep copy a linked list where each node has a next pointer and a random pointer to any node.
Design a data structure for Least Recently Used cache with O(1) get and put.