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@@ -49,3 +49,94 @@ The operations can be implemented with a Deque interface.
 * `removeFirst()` -> `remove(0)`
 * `addLast(x)` -> `add(size(), x)`
 * `removeLast()` -> `remove(size() - 1)`
+
+### USet
+
+The `USet` interface represents an unordered set of unique elements, which
+mimics a mathematical set. A `USet` contains `n` distinct elements; no
+element appears more than once; the elements are in no specific order. A
+`USet` supports the following operations:
+
+* `size()`: return the number, `n`, of elements in the set.
+* `add(x)`: add the element `x` to the set if not already present;
+* `remove(x)`: remove `x` from the set;
+* `find(x)`: find `x` in the set if it exists
+
+### SSet
+
+The `SSet` interface represents a sorted set of elements. An `SSet` stores elements
+from some total order so that any two elements x and y can be compared. In code
+examples, this will be done with a method called `compare(x, y)` in which:
+
+* < 0 if x < y
+* > 0 if x > y
+* = 0 if x == y
+
+An `SSet` supports the `size()` and `add()` and `remove()` methods with
+exactly the same semantics as in the `USet` interface. The difference
+between a `USet` and an `SSet` is in the `find(x)` method:
+
+> successor search: locate x in the sorted set;
+> find the small element y in the set such that y >= x.
+> return y or null if no such element exists.
+
+
+The extra functionality provided by a SSet usually comes with a price that
+includes both a larger running time and a higher implementation complexity.
+SSet implementations may have a `find(x)` running time of of logarithmic
+and a USet may have a running time of constant time.
+
+
+## Math Review
+
+### Exponentials and Logarithms
+
+The expression b^x denotes the number `b` raised to the power of `x`.
+
+* when x is negative, b^x = 1/(b^-x)
+* when x is 0, b^x = 1
+
+
+```text
+b^x = b * b * ... x b
+      |____________|
+            |
+         x times
+```
+
+```ruby
+b ** x = (x.times.inject(1) { |m, _| m * b }
+```
+
+```irb
+irb(main):001:0> 2 ** 10
+=> 1024
+irb(main):002:0> 10.times.inject(1) { |m, _| m * 2 }
+=> 1024
+```
+
+log b(k) deontes base-b logarithm of k. i.e b^x = k
+
+```text
+  log b(k) == b^x = k
+```
+
+```ruby
+irb(main):016:0> 2 ** 10
+=> 1024
+irb(main):017:0> Math.log2(1024)
+=> 10.0
+```
+
+An informal way to think about logarithms is to think of logb(k) as the number
+of times we have to divide k by b before the result is less than or equal to 1.
+
+For example, when one does binary search, each comparison reduces the number of
+possible answers by a factor of 2. This is repeated until there is at most one
+possible answer. Therefore the number of comparisons done by binary search when there
+are initially at most n + 1 possible answers is at most log2(n+1).
+
+Another logarithm that comes up several times in this book is the natural logarithm.
+Here we use the notation ln k to denote log e(k), where e -- Euler's constant -- is given by:
+
+![eulers constant](./img/eulers-constant.png)