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Unit 1. Sorting and Divide and ConquerPowerPoint Presentation

Unit 1. Sorting and Divide and Conquer

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### Unit 1.Sorting and Divide and Conquer

### Lecture 1 Introduction to Algorithm and Sorting

What is anAlgorithm?

- An algorithm is a computational procedure that takes some value, or a set of values, as input and produces some values, or a set of values, as output.
- So, it is a sequence of computational steps that transform the input into the output.
- Correction: sequence -> combination

Sequential and Parallel

- In sequential computation, an algorithm is a sequence of computational steps.
- However, it is not true in parallel computation.
- In this course, we study only algorithms in sequential computation.

Efficiency

- Running time from receiving the input to producing the output.

Running time

Insert Sort

Merge Sort

In the key outside of array.

How to calculate running time?

- Each “line” of pseudocode requires a constant time. (In RAM model)

and each time runs at most

4+3(j-1) lines.

This loop runs at most

j-1 times and each time

runs at most 3 lines.

Remark on Running Time

- Running time is a function of input size.
- In Turing machine model, the input size of sorting is

Divide and Conquer

- Divide the problem into subproblems.
- Conquer the subproblems by solving them recursively.
- Combine the solutions to subproblems into the solution for original problem.

How to Solve Recurrences

- Substitution method.
- Recursion-tree method.
- Master method.

Substitution Method

- Guess the form of the solution.
- Use math induction to find the constants and also show the solution works.

What we learnt in this lecture?

- How to calculate running time.
- How to solve recurrences.
- Insertion sort and Merge sort.
- Divide and conquer
- Lecture Notes give you key points in each lecture.
- You must read textbook after lectures in order to study well.

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