题解 | 校门外的树（boolean[] tree）

import java.util.Scanner;

// 注意类名必须为 Main, 不要有任何 package xxx 信息
public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int l = in.nextInt();
        int m = in.nextInt();
        int count = 0;
        boolean[] tree = new boolean[l + 1];
        for (int i = 0; i <= l; i++) {//初始化树值
            tree[i] = true;
        }
        for (int i = 0; i < m; i++) {
            int left = in.nextInt();
            int right = in.nextInt();
            for (int j = left; j <= right; j++) {//左右区间标记
                tree[j] = false;
            }
        }
        for (boolean num : tree) {//统计树
            if (num == true) {
                count++;
            }
        }
        System.out.println(count);
    }
}