C has nothing native for hashmap We need to write one ourselves.

C++ has unordered_map which is implemented via hashing. We will discuss treemaps in C++ as well later, which in practice seem to behave faster in C++.

unordered_map<int, int> A; // declares an empty map. O(1)

A.insert(key, value); // O(1) time on average

if (A.find(K) == A.end()) return null; // means that K does not exist in A. else return A[K]; // O(1) average case. Rare worst case O(n).

A.size() // O(1)

if (A.find(K) != A.end()) A.erase(A.find(K)); OR A.erase(K);

Now we come to one of the most popular data structures in Java, HashMap.

HashMap<Integer, Integer> A = new HashMap<Integer, Integer>(); // declares an empty map.

A.put(key, value); // O(1) time on average

A.get(K) // null if the key K is not present A.containsKey(K) tells if the key K is present. // Both operations O(1) average time. O(n) rare worst case

A.remove(K);

Python has dictionaries which serve the same purpose.

A = {}

A[key] = value // O(1) average time. O(n) worst case

A[K] // O(1) average, O(n) worst

len(A) // O(1)

del A[K] // O(1) average, O(n) worst

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