Hashing

from collections import defaultdict
counts = defaultdict(int)
len(counts)
for key in counts:

counts = {}
counts.get(key, 0 /* default value */)
dictionary.values() doesn't actually return a list, but actually a view object. We need to convert it to a list first.

dict is HashMap in Python - uses open addressing. key needs to be immutable.

Counting

sliding window is for max and prefix sum hash map is for counting.

• Python -> sort, reverse a.sort() and sorted(a) “True” “False”
• Algorithms convert to Set to check unique O(n)

any random access is O(1) add, remove, update, find.