本蒟蒻的代码有一点长，大佬们写的代码短多了

#include<bits/stdc++.h>
#define ll long long
using namespace std;
ll n,a[500005];
ll t1=0,t2=0;
int main(){
	cin>>n;
    for(int i=1;i<=n;i++) cin>>a[i];
    sort(a+1,a+1+n);
    for(int i=n;i>=1;i--){
        if(n%2==0){
            if(i%2==0) t1+=a[i];
            else if(i%2!=0) t2+=a[i];
        }
        if(n%2!=0){
            if(i%2==0) t2+=a[i];
            else if(i%2!=0) t1+=a[i];
        }
    }
    cout<<t1-t2;
    return 0; 
}