# CS 97SI: Introduction to Competitive Programming Contests

## Course Information and Announcements

• Instructor: Jaehyun Park

• Stanford ACM-ICPC Coaches: Jaehyun Park, Andy Nguyen, Jerry Cain

• (Added on 8/21/2013) This class was taught in 2011-12 Winter. I'm getting a lot of emails asking if I'm teaching it again, but there is no plan to offer the course at the moment.

## Practice Problems

All the problems below are from Peking Online Judge (POJ). Submissions should be made directly to the automated judging system.

Problems are classified into 10 different categories, and the lectures will cover essential algorithms and theoretical background for each particular category.

The numbers in parentheses represent the difficulty of the problems (0: easiest, 10: hardest). The difficulty rating is subjective; you might find a 5-rated problem easier than a 4-rated one. But do not attempt the challenge problems unless you are sure that you can solve all the easier ones.

## Common coding mistakes

• Not initializing variables

• Using 32-bit integers instead of 64-bit ones

• Using out-of-bound array indices

• Using a semicolon after a for loop

```for(i = 0; i < n; i++);
some code
```
• Reusing the same variable in nested for loops

```for(i = 0; i < 1000; i++)
for(i = 0; i < 10; i++)
some code
```
• Not using break in a switch-case statement (just don't use switch-case statements for programming contests)

• Not taking (very) small cases into account (e.g. )

• Writing arr[j][i] instead of arr[i][j], and similar errors

• Writing a given constant incorrectly (e.g. taking modulo 12345789 instead of 123456789)

• Bad parentheses in macro definition

```#define min(a, b) a<b?a:b // incorrect
#define min(a, b) (((a)<(b))?(a):(b)) // correct
```
• Writing cos(180) instead of cos(pi)

## If you get stuck…

• First, remember that you are not stuck unless you have spent more than a day on a single problem. It is perfectly normal to spend many hours on just coming up with the right algorithm.

• Read the problem statement again. You might have missed something very important.

• Carefully go through the lecture slides. Think about how to apply the basic algorithms covered in class to the problem.

• Discuss the problem with friends. Working in teams of size 2 or 3 is strongly recommended. As long as you don't copy-paste someone else's codes blindly, you can even share your (possibly buggy) codes.