题解 | 校门外的树

#include <iostream>
#include <vector>

using namespace std;
int main() {
	int L = 0, M = 0;
	cin >> L >> M;
	vector<int>arr(L+1, 0);
	vector<int>diff(L + 1, 0);
	int l, r;
	while (M--) {
		cin >> l >> r;
		diff[l] += 1;
		diff[r + 1] -= 1;
		if (l == 0) arr[0] = 1;
		for (int i = 1; i <= L; i++) {
			arr[i] = diff[i] + arr[i - 1];
		}
	}
	int count = 0;
	for (int i = 0; i <= L; i++) {
		if (arr[i] == 0) count++;
	}
	cout << count << endl;
}