In this slide, we solve the SPOJ problem: OLOLO, using std::unordered_set container from the C++ Standard Library

1. Level up your coding skills with
the C++ Standard Template
Library (STL):
SPOJ Problem: OLOLO
BY JOYJIT CHOUDHURY

2. OLOLO - Onotole needs your help
 Is accessible at - http://www.spoj.com/problems/OLOLO/

3. Problem Statement
 Onotole has a lot of pyani. Each pyani has a
number, writing on it.
 Pyanis with equal numbers are indistinguishable.
 Onotole knows everything, so, he knows that each
pyani appeared twice, and only one pyani is
unique. He wants to get вздръжни эффект, and he
needs the unique pyani.
 Given the list of pyanis denote which one of them
appeared once (it is guaranteed that
other pyanis appeared twice).

4. Input
 First line of input contains number of pyanis
N <= 500 000. Next N lines contain a single positive
integer 1 <= Pi <= 10^9.
Output
 Output one positive integer on pyani, which appeared
once.
Time limit: 0.134s
Source limit: 50000B
Memory limit: 1536MB

5. So, the idea is…
 There will be N numbers as input
 All the numbers come in pairs, except one of them
 We have to output that one number

6. So…
 There can be many different approaches to solving this problem
(read: using XOR)
 A solution using std::unordered_set will be demonstrated here
 For every number Pi, we check whether it’s already in the set
or not:
 If it’s not present, that means it’s the first occurrence of the
number. Thus, we insert it into the set
 If it is present, that means it has occurred once before. So, we
remove it from the set

7. In the End
 There will be just one number left in the
unordered_set
 The one which didn’t come in pair
 The one we need to output

8. Code: I

9. Code: II

11. Solution Accepted!