P1047 [noip2005 popularization group t2] tree outside the school gate

【 Title source 】
https://www.luogu.com.cn/problem/P1047
https://www.acwing.com/problem/content/description/424/

【 Title Description 】
The length of a school gate is L There are a row of trees on the road , The interval between every two adjacent trees is 1 rice .
We can think of the road as a number axis , One end of the road is on the number axis 0 The location of , In the other end L The location of ; Every integer point on the number axis , namely 0,1,2,……,L, They all have a tree .
Because there are some areas on the road that will be used to build subways .
These areas are represented by their starting and ending points on the number axis .
It is known that the coordinates of the starting point and the ending point of any area are integers , There may be overlaps between areas .
Now let's put the trees in these areas （ Includes two trees at the end of the area ） Removal .
Your task is to calculate how to move all these trees away , How many trees are there on the road .

【 Input format 】
The first line of the input file has two integers L and M.L Represents the length of the road ,M Represents the number of regions ,L and M Separated by a space .
Next M Each line contains two different integers  u,v, Separated by a space , Represents the coordinates of the starting point and the ending point of an area .

【 Output format 】
The output file contains one line , This line contains only one integer , Represents the number of trees remaining on the road .

【 Data range 】
about  20% The data of , Ensure that there is no overlap between areas .
about 100% The data of , Guarantee 1≤L≤10000,1≤M≤100,0≤u≤v≤L.

【 Algorithm code 】

#include <bits/stdc++.h>
using namespace std;

const int maxn=10005;

int L,M;
int f[maxn];

int main() {
	scanf("%d%d",&L,&M);
	while(M--) {
		int le,ri;
		scanf("%d%d",&le,&ri);
		for(int i=le; i<=ri; i++) f[i]=1;
	}

	int ans=0;
	for(int i=0; i<=L; i++){
		if(f[i]==0) ans++;
	}		

	printf("%d\n",ans);

	return 0;
}

/*
in:
500 3
150 300
100 200
470 471

out:
298
*/

【 reference 】
https://www.acwing.com/solution/content/3097/
https://www.luogu.com.cn/problem/solution/P1047