0704. Binary Search -- easy
0027. Remove Element -- easy
0001. Two Sum -- easy
0242. Valid Anagram -- easy
0349. Intersection of Two Arrays -- easy
1002. Find Common Characters -- easy
0202. Happy Number -- easy
0454. 4 Sum 2 -- medium
0383. Ransom Note -- easy
0015. 3 Sum -- medium
0018. 4 Sum -- medium
1. String and Char
2. Vector
3. Set
4. Map
5. Algorithms Library
6. Object Oriented Programming