第一次接触算法也是第一次用洛谷，现在很懵逼 输入以下代码

#include <iostream>
using namespace std;
void MINGrandomnumber (int a[],int N)
{
    int j = 0, M = 0,m=0,x;
    cin >> N;
    int* r = new int [m+1];
    while(cin >> x)
    {
        a[j] = x;
        j++;
    }
    for(int i =1;i<N;i++)
    {
        for(int j =i-1;j>=0;j--)
        {
            if (a[i] == a[j])
            a [i] =0;
        }
    }
    for(int i = 0;i<N;i++)
    {
        if(a[i] != 0) 
        {
          r[m] = a[i];
          m++;M++;
        }
    }
    for(int i =m;i >=0;i--)
    {
        for(int j =0;j<i;j++)
        {
            if(r[i] >r[i+1])  swap(r[i],r[i+1]);
        }
    }
    cout << M << endl;    
    int fu = 0;
    while(fu <m)
    {
        cout << r[fu] << " ";
        fu++;
    }
    
    
}
`提交后如下报错，凭借我稀烂的英语，看出好像是编译器报错。不明白问题出在哪，希望能有大佬帮我看看``

No valid executable file was produced by the compiler /usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/8/../../../x86_64-linux-gnu/Scrt1.o: in function _start': (.text+0x20): undefined reference to main' collect2: 错误：ld 返回 1