adding lecture 3 notes

lectures/03_classes_I/README.md

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+# Lecture 3 --- Claases
+
+- Classes in C++; Types and defining new types; Example: A Date class;
+- Class declaration: member variables and member functions; Using the class member functions;
+- Member function implementation; Class scope; Classes vs. structs; Designing classes;
+- Defining a custom sort of class instances.
+
+## 3.1 Types and Defining New Types	
+
+## 3.2 C++ Classes
+
+## 3.3 Class scope notation
+
+## 3.4 Constructors
+
+## 3.5 Member Functions
+
+## 3.6 Constant member functions	
+
+## 3.7 Classes vs. structs
+
+## 3.8 Designing and implementing classes
+
+## 3.9 Providing Comparison Functions to Sort
+
+## 3.10 Operator Overloading
+
+## 3.11 Exercises
+
+- [Leetcode problem 8: String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/)
+- [Leetcode problem 211: Design Add and Search Words Data Structure](https://leetcode.com/problems/design-add-and-search-words-data-structure/)
+- [Leetcode problem 1662: Check If Two String Arrays are Equivalent](https://leetcode.com/problems/check-if-two-string-arrays-are-equivalent/)