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--- courses:cs211:winter2018:journals:patelk:chapter2 [2018/01/19 18:45] – [Personal Thoughts] patelk
+++ courses:cs211:winter2018:journals:patelk:chapter2 [2018/01/28 20:18] (current) – [2.5 A More Complex Data Structure: Priority Queues] patelk
@@ Line 211: / Line 211: @@
 Readability: 6
 Interesting: 6
+===== 2.5 A More Complex Data Structure: Priority Queues =====
+**The Problem**
+  * Priority Queue: Elements have a priority value (key), and each time we need to select an element from S, we want to take the one with highest priority.
+          - Set of elements S
+          - Smaller keys represent higher priority
+  * Good for processing real-time events such as the scheduling of processes on a computer
+  * Priority Queue Operations: any sequence of priority queue operations that results in the sorting of n numbers -> **O(nlogn)**
+**A Data Structure for Implementing a Priority Queue**
+  * List: have a pointer to the min; adding new elements is easy, but extracting the minimum requires O(n) scan to find the new minimum
+  * Sorted Array: binary search to find position of insertion + shifting all elements -> O(nlogn)
+  * Sorted Doubly Linked List: insertions are O(1), but O(n) to find position of insertion
+__The Heap:__ balanced binary tree; root + nodes with up to two children; key of any element is at least as large as the key of the parent node
+  * For a heap with bound N, we can use an array
+  * H[1] is the root
+  * leftChild(i) = 2i
+  * rightChild(i) = 2i+1
+**Implementing the Heap Operations**
+  * Identifying Minimal Element: **O(1)**
+  * Adding an Element: add element to the final position, then perform heapify-up recursively -> **O(logn)**
+{{:courses:cs211:winter2018:journals:patelk:heapify-up.png?nolink&400|}}
+  * Deleting an Element: move element w in position n to position i; key of element w may either be too small or too bit -> **O(logn)**
+  *   * If key is too small: use heapify-up to reestablish order
+  *   * If key is too large: use heapify-down to sway the element with one of its children
+{{:courses:cs211:winter2018:journals:patelk:heapify-down.png?nolink&400|}}
+**Implementing Priority Queues with Heaps**
+  * __StartHeap(N):__ returns an empty heap that is set up to store at most N elements -> **O(n)**
+  * __Insert(H,v):__ inserts item v into heap H -> **O(logn)**
+  * __FindMin(H):__ identifies the min -> **O(1)**
+  * __Delete(H,i):__ deletes element in position i -> **O(logn)**
+  * __ExtractMin(H):__ identifies and deletes minimum key element -> **O(logn)**
+----
+==== Personal Thoughts ====
+This section was pretty straightforward and easy to follow. I think going over the concepts in class before reading this section of the textbook was helpful in clarifying things that maybe would have been otherwise confusing. I appreciated the summary of the operation run times as these can sometimes be difficult to recall.
+Readability: 9
+Interesting: 6