[Cqoi2014]危桥 （两遍网络流）

题目链接

 #include <bits/stdc++.h>
 using namespace std;
 typedef long long ll;
 inline int read()
 {
     int x=,f=;char ch=getchar();
     while(ch<''||ch>''){if(ch=='-')f=-;ch=getchar();}
     while(ch>=''&&ch<=''){x=x*+ch-'';ch=getchar();}
     return x*f;
 }

 /********************************************************************/

 #define inf 0xffffff
 #define T 2001
 const int maxn = 2e6+;
 const int Maxn = 2e3+;
 int a, b;
 int ans1, ans2;
 int head[Maxn], q[Maxn], dis[Maxn], from[Maxn];
 bool vis[Maxn];

 struct node
 {
     int to, from, Next;
     int v, c;
 }e[maxn];
 int cnt = ;

 int gcd(int x, int y){
     if(y == ) return x;
     else return gcd(y, x%y);
 }

 void add_edge(int u, int v, int w, int c){
     e[++cnt].to = v; e[cnt].from = u; e[cnt].Next = head[u]; head[u] = cnt;
     e[cnt].v = w; e[cnt].c = c;
 }

 void insert(int u, int v, int w, int c){
     add_edge(u, v, w, c);
     add_edge(v, u, , -c);
 }

 //是否满足条件
 bool check(int x, int y){
     if(x < y) swap(x, y);
     int t = int(sqrt(x*x-y*y));
     return (gcd(y, t) ==  && x*x-y*y == t*t);
 }

 bool spfa(){
     for(int i = ;i <= T;i++){
         dis[i] = -inf;
     }
     int t = , w = ;
     dis[] = ; q[] = ; vis[] = ;
     while(t != w){
         int now = q[t]; t++;
         if(t == T) t = ;
         for(int i = head[now];i;i = e[i].Next){
             if(e[i].v && e[i].c+dis[now] > dis[e[i].to]){
                 dis[e[i].to] = e[i].c + dis[now];
                 from[e[i].to] = i;
                 if(!vis[e[i].to]){
                     vis[e[i].to] = ;
                     q[w++] = e[i].to;
                     if(w == T) w = ;
                 }
             }
         }
         vis[now] = ;
     }
     if(dis[T] == -inf) return false;
     return true;
 }

 void dfs(){
     int x = inf;
     for(int i = from[T];i;i = from[e[i].from]){
         x = min(e[i].v, x);
     }
     for(int i = from[T];i;i = from[e[i].from]){
         ans2 += x*e[i].c;
         e[i].v -= x;
         e[i^].v += x;
     }
 }

 int main(){
     a = read(); b = read();
     for(int i = a;i <= b;i++){
         for(int j = a;j <= b;j++){
             if(check(i, j) && i != j){
                 insert(i, j+, , i+j);
             }
         }
     }
     for(int i = a;i <= b;i++){
         insert(, i, , );
         insert(i+, T, , );
     }
     while(spfa()) dfs();
     for(int i = ;i <= cnt;i += ){
         if()
     }
     return ;
 }