Java 代码

//类似LeetCode合并区间……这个简单一点
//关键是去重，把每棵树扔到set里去就行，hashset就行
//因为不用排序，然后就没然后了……
import java.util.*;

public class Main {
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int l = in.nextInt();
        int m = in.nextInt();
        int[][] a = new int[m][2];
        for (int i = 0; i < m; i++) {//输入区间
            a[i][0] = in.nextInt();
            a[i][1] = in.nextInt();
        }
        Set<Integer> set = new HashSet<>();
        for (int[] i : a) {
            for (int j = i[0]; j <= i[1]; j++)//每棵树扔进去，去重即可
                set.add(j);
        }
        System.out.println(l + 1 - set.size());
    }
}