202112-1 序列查询

#include <bits/stdc++.h>
using namespace std;
int main(){int n, N;cin >> n >> N;vector<int> A(n + 1,0);vector<int> fa(N + 1,0);for(int i = 1; i <= n; i++){cin >> A[i];}int j = 1;int sum = 0;for(int i = 1; i <= N; i++){fa[i] = fa[i - 1];if(j < A.size() && i >= A[j]){fa[i]++;j++;}if(i == N)break;sum += fa[i];}cout << sum;return 0;
}