有N个正整数，需要从中选出一些数，使这些数的和最大。
若两个数a,b同时满足以下条件，则a,b不能同时被选
1:存在正整数C，使a*a+b*b=c*c
2:gcd(a,b)=1

n<=3000。

网络流

 #include <cmath>

 #include <cstdio>

 #include <cstring>

 typedef long long lnt;

 const int siz = ;

 const int inf = ;

 int n;

 int a[siz];

 int b[siz];

 int tot;

 int s, t;

 int hd[siz];

 int to[siz];

 int fl[siz];

 int nt[siz];

 inline void add(int u, int v, int f)

 {

     nt[tot] = hd[u]; to[tot] = v; fl[tot] = f; hd[u] = tot++;

     nt[tot] = hd[v]; to[tot] = u; fl[tot] = ; hd[v] = tot++;

 }

 int dep[siz];

 inline bool bfs(void)

 {

     static int que[siz];

     static int head, tail;

     memset(dep, , sizeof(dep));

     head = , tail = ;

     que[tail++] = s;

     dep[s] = ;

     while (head != tail)

     {

         int u = que[head++], v;

         for (int i = hd[u]; ~i; i = nt[i])

             if (!dep[v = to[i]] && fl[i])

                 dep[que[tail++] = v] = dep[u] + ;

     }

     return dep[t];

 }

 int cur[siz];

 int min(int a, int b)

 {

     return a < b ? a : b;

 }

 int dfs(int u, int f)

 {

     if (u == t || !f)

         return f;

     int used = , flow, v;

     for (int i = cur[u]; ~i; i = nt[i])

         if (dep[v = to[i]] == dep[u] +  && fl[i])

         {

             flow = dfs(v, min(f - used, fl[i]));

             used += flow;

             fl[i] -= flow;

             fl[i^] += flow;

             if (used == f)

                 return f;

             if (fl[i])

                 cur[u] = i;

         }

     if (!used)

         dep[u] = ;

     return used;

 }

 inline int maxFlow(void)

 {

     int maxFlow = , newFlow;

     while (bfs())

     {

         for (int i = s; i <= t; ++i)

             cur[i] = hd[i];

         while (newFlow = dfs(s, inf))

             maxFlow += newFlow;

     }

     return maxFlow;

 }

 inline lnt sqr(lnt x)

 {

     return x * x;

 }

 int gcd(int x, int y)

 {

     return y ? gcd(y, x % y) : x;

 }

 inline bool check(int x, int y)

 {

     if (gcd(x, y) != )

         return false;

     lnt t = sqr(x) + sqr(y);

     if (sqr(sqrt(t)) != t)

         return false;

     return true;

 }

 signed main(void)

 {

     scanf("%d", &n);

     for (int i = ; i <= n; ++i)

         scanf("%d", a + i);

     for (int i = ; i <= n; ++i)

         b[i] = a[i];

     s = , t = n + ;

     memset(hd, -, sizeof(hd));

     for (int i = ; i <= n; ++i)

         if (a[i] & )

             add(s, i, b[i]);

         else

             add(i, t, b[i]);

     for (int i = ; i <= n; ++i)

         for (int j = ; j <= n; ++j)

             if (a[i] & )if (!(a[j] & ))

                 if (check(a[i], a[j]))

                     add(i, j, inf);

     int sum = ;

     for (int i = ; i <= n; ++i)

         sum += b[i];

     printf("%d\n", sum - maxFlow());

 }