Gain insights into competitive programming, explore C++ skills with theory, code samples, practice problems, and master faster implementation for contests like ACM ICPC, Google CodeJam, and HackerCup.

Competitive programming can be a great way to build out your programming skills, get on any major company’s radar, and earn a little extra cash along the way.

In this course, you will learn to prepare for competitive programming contests like ACM ICPC, Google CodeJam, Facebook HackerCup, and many more.

Each topic is broken down with a healthy mix of theory, code samples, step-by-step solved sample problems, illustrations, useful practice problems, and tips and tricks for faster implementation.

You will need some solid C++ foundations coming into this course, but by the end it, you will be confident enough in your C++ skills to take home the win.

Competitive Programming in C++: The Keys to Success

# Problem statement
There are $N$ different types of balls. There are $k_1$ balls of type 1, $k_2$ balls of type $2$, and so on to $k_n$. How many ways can you choose $2$ balls that are of different types?

**Input format**

The first line contains one positive integer N $(1 \leq N \leq 10^5)$ – the number of types of balls.

The second line contains a sequence $k_1, k_2, ..., k_N$ where $(1 \leq k_i \leq 10^5)$ - the count of balls of each type.

**Output format**

Print a single integer to answer the problem.


---

# Sample
**Input 1**
```
3
2 1 1
```
**Output 1**
```
5
```

**Input 2**
```
4
1 2 2 2
```
**Output 2**
```
18
```

---

# Solution
We will subtract the non-desirable number of ways from the totals,

Total number of ways - Pick 2 from ($k_1 + k_2 + ... + k_n$) balls

$A = {k_1 + k_2 + ... + k_n \choose 2}$

Non-desirable case - when both balls are of the same type. The number of ways to pick that is:

$B= {k_1 \choose 2} + {k_2 \choose 2} + ... + {k_n \choose 2}$

The answer is just $A-B$

**Note:** Use `long long int` since the result can overflow `int`.


In this lesson, we'll discuss a solved PnC Problem.


Introduction

Complexity Analysis

Number Theory

Arrays and Vectors

Sieve of Eratosthenes

Strings

Sorting

Linked List

Stack

Queue

Binary Tree

2 Pointers

Heap

Binary Search Tree

Balanced Binary Search Tree

Course Conclusion

Solved Problem - PnC

Problem statement