Why does my code doesn't print the correct minimum difference?

Question

I'm currently trying to solve the horse-racing game on Codingame. The problem statement is as follows:

Input

Line 1: Number N of horses

The N following lines: the strength Pi of each horse. Pi is an integer.

Output

The difference D between the two closest strengths. D is an integer greater than or equal to 0.

I'm using a std::set, and my algorithm is quite easy but for some reason it fails 2/8 of the submission validators. I don't have access to the actual test data, but according to the test names, the code below fails for

• all horses same strength
• horses in disorder

Why does my code fail on that input?

#include <iostream>
#include <set>

using namespace std;

int main()
{
    int N;
    cin >> N; cin.ignore();

    set<int> power;

    for (int i = 0; i < N; i++)
    {
        int Pi;
        cin >> Pi; cin.ignore();
        power.insert(Pi);
    }

    int minDiff = 10000000;

    auto i = power.begin();

    while (i != power.end())
    {
        minDiff = min(minDiff, abs(*i - *(++i)));
    }
    
    cout << minDiff << endl;
}

Answer 1

Your solution fails if any two horses have the same strength:

std::set<int> horses;
horses.insert(1);
horses.insert(2);
horses.insert(1);

// your algorithm returns 1, but it should be 0

Also, you have undefined behaviour. What happens if ++i is power.end() here?

minDiff = std::min(minDiff, abs(*i - *(++i)));

Use two iterators instead, and check for the special case where you have only one kind of horse if you want to use std::set.