#include <stdio.h>
int n;
int main()
{
    scanf("%d",&n);
    long long ans = n;
    if(ans%2==1)
    {
    	ans = ans+1;
    	ans/=2;
    	ans *= n;
	}
	else 
	{
		ans /=2;
		ans *= (n+1); 
	}
	for(int i = 0,x;i < n - 1;i++)
	{
		scanf("%d",&x);
		ans -= x;
	}
	printf("%lld\n",ans);
	return 0;
}

异或：

#include<bits/stdc++.h>
using namespace std;
const int N=1e8+9,INF=0x3f3f3f3f;
unsigned int n,x,ans;//记得开unsigned int
int main(){
	cin>>n;
	for(int i=1;i<=n-1;i++){
		cin>>x;
		ans^=x;
		ans^=i;
	}
	ans^=n;
	cout<<ans;
	return 0;
}

求和：

#include<bits/stdc++.h>
using namespace std;
const int N=1e5+9,INF=0x3f3f3f3f;
long long n,x,sum;//记得开long long
int main(){
	cin>>n;
	for(int i=1;i<=n-1;i++){
		cin>>x;
		sum+=x;
	}
	cout<<(1+n)*n/2-sum;
	return 0;
}

给个点赞吧

大大的位运算

#include<queue>
#include<math.h>
#include<stdio.h>
#include<iostream>
#include<vector>
#include<iomanip>
#include<string.h>
#include<algorithm>
#include<cmath>
#include<cstdio>
#include<utility>
#include<cstring>
#include<stack>
#include<fstream>
#include <sstream>
#include<string>
#include<stdlib.h>
using namespace std;
#define ll long long
const int N = 1e6 + 10;
const int INF = 0x3f3f3f3f;
unsigned int n , x , ans;
int main()
{
    cin >> n;
    for ( int i = 1 ; i <= n - 1 ; i++ )
    {
        cin >> x;
        ans ^= x;
        ans ^= i;
    }
    ans ^= n;
    cout << ans;
    return 0;
}

/*
int       %d
longlong  %lld
float     %f
double    %lf
char      %c
char[]    %s
'a'=97
'z'=122
'A'=65
'Z'=90
'0'=48
'9'=57
*/
#include <cstdio>
using namespace std;
const int N=1e7+10;
const int INF=0x3f3f3f3f;
const double pi=3.1416;
int n,x,ans;
int main(){
	scanf("%d",&n);
	for(int i=1;i<n;i++){
		scanf("%d",&x);
		ans^=x;
	}
	for(int i=1;i<=n;i++){
		ans^=i;
	}
	printf("%d",ans);
return 0;
}

#include<stdio.h>//节省空间
int main(){
	int n;
	scanf("%d",&n);
	int ans=n;
	for(int i=1,x;i<n;i++){
		scanf("%d",&x);
		ans=ans^x^i;//^是异或
	}
	printf("%d",ans);
	return 0;
}

#include <stdio.h>
int main()
{
	int n,ans;
	scanf("%d",&n);
	ans = n;
	for(int i = 1 , x ; i < n ; i++)
	{
		scanf("%d",&x);
		ans = ans ^x ^ i;
	}
	printf("%d\n",ans);
}