差分约束

#include <iostream>
#include <vector>
#include <algorithm>
#include <queue>
using namespace std;
const int MAXN=10010;
typedef struct node{
    int v;
    int w;
}node;

vector<node> edge[MAXN];
int vis[MAXN],d[MAXN],n=0;

void spfa(int src){
    for(int i=0;i<=n;i++) d[i]=-1,vis[i]=0;
    d[src]=0;
    queue<int> q;
    q.push(src);
    while(!q.empty()){
        int u=q.front();
        q.pop();
        vis[u]=0;
        for(int i=0;i<edge[u].size();i++){
            node x=edge[u][i];
            int tmp=d[u]+x.w;
            if(tmp>d[x.v]){
                d[x.v]=tmp;
                if(!vis[x.v]){
                    vis[x.v]=1;
                    q.push(x.v);
                }
            }
        }
    }
}

int main(){
    int m;
    cin>>m;
    while(m--){
        int a,b;
        scanf("%d%d",&a,&b);
        a++,b++;
        edge[a-1].push_back(node{b,2});
        edge[a-1].push_back(node{b-1,1});
        n=max(n,b);
    }
    for(int i=0;i<n;i++) edge[i].push_back(node{i+1,0});
    spfa(0);
    cout<<d[n];
}