题解 | 校门外的树

import java.util.Scanner;

public class Main {

	public static void main(String[] args) {
		// TODO Auto-generated method stub
		Scanner scanner=new Scanner(System.in);
		int L=scanner.nextInt();
		int m=scanner.nextInt();
		int a[]=new int[L+1];
		for (int i = 0; i < m; i++) {
			int l=scanner.nextInt();
			int r=scanner.nextInt();
			for (int j = l; j <= r; j++) {
				a[j]=1;
			}
		}
		int count=0;
		for (int i = 0; i < a.length; i++) {
			if(a[i]==0) {
				count++;
			}
		}
		System.out.println(count);
		

	}

}

本题其实不难，想到用数组赋值来做就好了，我一年前做这题是还在想着用数学计算做题，那就很麻烦。设置一个数组，把要修地铁的区域全部设为1，最后遍历统计非1的个数即可