adding lecture 4 notes

lectures/04_pointers/README.md

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+# Lecture 4 --- Pointers, Arrays, & Pointer Arithmetic
+
+- Pointers store memory addresses.
+- They can be used to access the values stored at their stored memory address.
+- They can be incremented, decremented, added, and subtracted.
+- Dynamic memory is accessed through pointers.
+- Pointers are also the primitive mechanism underlying vector iterators, which we have used with std::sort and
+will use more extensively throughout the semester.
+
+## 4.1 Pointer Example
+
+- Consider the following code segment:
+
+```cpp
+float x = 15.5;
+float *p; /* equiv: float* p; or float * p; */
+p = &x;
+*p = 72;
+if ( x > 20 )
+cout << "Bigger\n";
+else
+cout << "Smaller\n";
+```
+
+The output is Bigger
+because x == 72.0. What’s going on?
+
+## 4.2 Exercises
+
+- [Leetcode problem 905: Sort Array By Parity](https://leetcode.com/problems/sort-array-by-parity/). Solution: [p905_sortarraybyparity.cpp](../../leetcode/p905_sortarraybyparity.cpp)
+- [Leetcode problem 977: Squares of a Sorted Array](https://leetcode.com/problems/squares-of-a-sorted-array/). Solution: [p977_sortedsquare.cpp](../../leetcode/p977_sortedsquare.cpp)
+- [Leetcode problem 1051: Height Checker](https://leetcode.com/problems/height-checker/). Solution: [p1051_heightchecker.cpp](../../leetcode/p1051_heightchecker.cpp)