Stack & Queue & Hash Map

Stack

Queue

Hash Map

seq = 'abcdabcd'
hash_map = {}
for c in seq:
	hash_map[c] = hash_map.get(c, 0) + 1

• 242: Valid Anagram

• 349: Intersection of Two Arrays

• 202: Happy Number

• 1: Two Sum

• 454: 4Sum II