建立矛盾边，2-SAT
1 #include<bits/stdc++.h> 2 #define Pii pair<int,int> 3 #define fi first 4 #define se second 5 #define maxn 1010 6 const int MAXN = 20020; 7 const int MAXM = 100010; 8 using namespace std; 9 int r; 10 struct Edge { 11 int to,next; 12 }edge[MAXM]; 13 int head[MAXN],tot; 14 void init(){ 15 tot = 0; 16 memset(head,-1,sizeof(head)); 17 } 18 void addedge(int u,int v){ 19 edge[tot].to = v; 20 edge[tot].next = head[u]; 21 head[u] = tot++; 22 } bool vis[MAXN];//染色标记，为true表示选择 23 int S[MAXN],top;//栈 24 bool dfs(int u){ 25 if(vis[u^1])return false; 26 if(vis[u])return true; 27 vis[u] = true; 28 S[top++] = u; 29 for(int i = head[u];i != -1;i = edge[i].next) 30 if(!dfs(edge[i].to)) 31 return false; 32 return true; 33 } 34 bool Twosat(int n){ 35 memset(vis,false,sizeof(vis)); 36 for(int i = 0;i < n;i += 2){ 37 if(vis[i] || vis[i^1]) 38 continue; 39 top = 0; 40 if(!dfs(i)){ 41 while(top) 42 vis[S[--top]] = false; 43 if(!dfs(i^1)) return false; 44 } 45 } 46 return true; 47 } 48 bool check(int x,int y){ 49 return (abs(y-x)<=2*r); 50 } 51 Pii seg[maxn]; 52 int main(){ 53 int n,m,l; 54 int u,v; 55 cin>>n>>r>>l; 56 for(int i=0;i<l;i++){ 57 scanf("%d%d",&seg[i].fi,&seg[i].se); 58 } 59 init(); 60 for(int i=0;i<l;i++){ 61 for(int j=i+1;j<l;j++){ 62 if(i==j) continue; 63 if(seg[i].se==seg[j].se&&check(seg[i].fi,seg[j].fi)){ 64 addedge((2*i)^1,2*j); 65 addedge((2*j)^1,2*i); 66 } 67 if(seg[i].fi==seg[j].fi&&check(seg[i].se,seg[j].se)){ 68 addedge(2*j,(2*i)^1); 69 addedge(2*i,(2*j)^1); 70 } 71 } 72 } 73 if(Twosat(2*l)) cout<<"YES"; 74 else cout<<"NO"; 75 return 0; 76 }