MnZn 60pts 求助

#include<iostream>
#include<algorithm>
#include<set>
const int sz = 1e3 + 10;
int arr[sz << 1], xpp, res[sz << 1];
std::pair<int, int> modify[sz];
int main() {
    std::ios::sync_with_stdio(false);
    std::cin.tie(nullptr);
    int n, m;
    std::cin >> n >> m;
    for (int i = 1, a, b; i <= m; i++)
        std::cin >> a >> b, modify[i] = std::make_pair(a, b), arr[++xpp] = a, arr[++xpp] = b;
    std::sort(arr + 1, arr + xpp + 1);
    int f = std::unique(arr + 1, arr + xpp + 1) - arr;
    for (int i = 1; i <= m; i++)
        modify[i] = std::make_pair(std::lower_bound(arr + 1, arr + f, modify[i].first) - arr,
                                   std::lower_bound(arr + 1, arr + f, modify[i].second) - arr);
    for (int i = 1; i <= m; i++)
        for (int j = modify[i].first; j <= modify[i].second; j++) res[j] = i;
    std::set<int> ans;
    for (int i = 1; i < f; i++)
        ans.insert(res[i]);
    std::cout << ans.size();
    return 0;
}