STL+数学解法

第一篇题解，抢沙发

#include<bits/stdc++.h>
using namespace std;
unordered_set<int> nums;
int main() {
    int n,k; cin >> n >> k;
    for(int i = 0;i < n;i++){
        int x; cin >> x;
        nums.insert(x);
    }
    long long total = (long long)(k) * (long long)(k + 1) / 2;
    long long sum = 0;  
    for(int i : nums){
        if(i <= k) sum += i;
    }
    cout << total - sum;
    return 0;
}