解法一：暴力枚举

#include <iostream>
using namespace std;
const int N = 10010;

bool tree[N];

int main(){
    int L,M;
    cin>>L>>M;
    for(int i=0;i<M;i++){
        int l,r;
        cin>>l>>r;
        for(int i=l;i<=r;i++)   tree[i] = true;
    }

    int res = 0;
    for(int i=0;i<=L;i++)
        if(tree[i] == false)
            res++;

    cout<<res<<endl;
    return 0;
}

算法2:区间合并

#include <iostream>
#include <algorithm>
using namespace std;
const int N = 110;
#define x first
#define y second

typedef pair<int,int> PII;

PII q[N];
int L,M;

int main(){
   cin>>L>>M;
   for(int i=0;i<M;i++) cin>>q[i].x>>q[i].y;
   //按左端点大小排序
   sort(q, q+M);    
   int sum = 0;
   int st=0,ed = -1;
   for(int i=0;i<M;i++){
       if(ed<q[i].x){
        sum += ed-st+1;
        st = q[i].x, ed = q[i].y;
       }
       else 
        ed = max(ed,q[i].y);
   }

   sum += ed-st+1;
   cout<<L+1-sum<<endl;
   return 0;
}