Your language's built-in collections and their costs

You build each structure once to understand it. After that, use your language's own versions for everything. Learn exactly which type to reach for and what each call costs, because hidden O(n) calls are the most common cause of slow solutions.

Know your dynamic array, hash map, hash set, deque, heap and sorted map (a map that keeps its keys in order). Add the handy extras too, such as Python's Counter and defaultdict, Java's ArrayDeque and TreeMap, or C++'s unordered_map and map.

Write a one-page table of the operations you use most, with the Big-O of each, and keep it beside you while you practise.