Codeforce問題の解決

Little Petya very much likes gifts. Recently he has received a new laptop as a New Year gift from his mother. He immediately decided to give it to somebody else as what can be more pleasant than giving somebody gifts. And on this occasion he organized a New Year party at his place and invited n his friends there.

If there's one thing Petya likes more that receiving gifts, that's watching others giving gifts to somebody else. Thus, he safely hid the laptop until the next New Year and made up his mind to watch his friends exchanging gifts while he does not participate in the process. He numbered all his friends with integers from 1 to n. Petya remembered that a friend number i gave a gift to a friend number pi. He also remembered that each of his friends received exactly one gift.

Now Petya wants to know for each friend i the number of a friend who has given him a gift.

Now, before discussing the solution, I first need to address a mistake in the description in the input of the question as given on Codeforces. In the input description, it says that:

the i-th number is pi — the number of a friend who gave a gift to friend number i.

This is not how the input is given in the various test cases. On closer inspection this is very similar to the description of the expected output of the questions:

i-th number should equal the number of the friend who gave a gift to friend number i.

So, now the natural question you'll be asking is "What exactly is the input format?"

If we re-examine the description of the Problem Statement above, we see the line:

He numbered all his friends with integers from 1 to n. Petya remembered that a friend number i gave a gift to a friend number pi.

Eureka! This is the correct description of the input format! We can verify this as well with the given sample examples. Let's try that now.

Alright, so now we know how to interpret out input and process it to get our output. To be extra sure we have also verified our logic over the given examples as well. Let's now translate this logic into code. For this I'll be using modern C++ (C++11 and beyond), so that it's easier to get the big picture logic which is implemented in the code.

Another interesting reason to use modern C++ is that some of its features make the code more readable and similar looking to Python code (many people consider this a plus).

/**
 * @file 136A-Presents.cpp
 * @author Rishit Chaudhary (@rishitc)
 * @version 1.0
 * @date 2021-06-23
 * 
 * @copyright Copyright (c) 2021
 * 
 */

#include <iostream>

int main()
{
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);
    int n;
    int p;

    std::cin >> n;

    int ans[n];

    for (int i = 1; i < n + 1; ++i)
    {
        std::cin >> p;

        ans[p - 1] = i; // Zero index correction for p
    }

    for (int i = 0; i < n; ++i)
    {
        std::cout << ans[i] << " ";
    }
    std::cout << "\n";
}