04.findTheMissingNumber.cpp

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

typedef vector <int> vi;
typedef pair< int ,int > pii;
#define endl "\n"
#define sd(val) scanf("%d",&val)
#define ss(val) scanf("%s",&val)
#define sl(val) scanf("%lld",&val)
#define debug(val) printf("check%d\n",val)
#define all(v) v.begin(),v.end()
#define PB push_back
#define MP make_pair
#define FF first
#define SS second
#define ll long long
#define MOD 1000000007
#define clr(val) memset(val,0,sizeof(val))
#define what_is(x) cerr << #x << " is " << x << endl; 
#define OJ \
    freopen("input.txt", "r", stdin); \
    freopen("output.txt", "w", stdout);
#define FIO ios_base::sync_with_stdio(false);cin.tie(NULL);cout.tie(NULL);

// can be large sum. might need to mod with 10^9 + 7 (same w XOR)
int missingNumberSUM(vector<int>& arr) {
    int n = arr.size();
    int sum = (n + 1) * (n + 2) / 2;
    for(auto &itr : arr) {
        sum -= itr;
    }
    return sum;
}

int missingNumberXOR(vector<int>& arr) {
    int n = arr.size();
    int res = arr.size() + 1;
    for(int i = 0; i < arr.size(); i++) {
        res ^= (i + 1);
        res ^= arr[i];
    }
    return res;
}
int main() {
    FIO
    OJ
    int size;
    cin >> size;
    vector<int> arr(size);  
    for(auto &it : arr) {
        cin >> it;
    }
    cout << missingNumberXOR(arr);
    return 0;
}