题解 | 剩下的树

#include <iostream>
#include <vector>
#include <numeric>

using std::cout;
using std::cin;
using std::endl;
using std::vector;
using std::accumulate; // 进行求和的函数

int main() {
    int L, M;
    cin >> L >> M;
    vector<int> vec;
    for (int i = 0; i <= L; i++)
        vec.push_back(1);
    for (int i = 0; i < M; i++) {
        int start;
        int end;
        cin >> start >> end;
        for (int j = start; j <= end; j++)
            vec[j] = 0;
    }
    cout << accumulate(vec.begin(), vec.end(), 0) << endl;

    return 0;
}