C++ :

#include <bits/stdc++.h>
using namespace std;

int n,a[100100],x;
int main() {
	cin>>n;
	for(int i = 1;i <= n;i++){
		cin>>x;
		a[x]++;
	}
	
	for(int i = 1;i <= n;i++){
		if(a[i] == 0) cout<<i<<" ";
	}
}