贪心 Codeforces Round #297 (Div. 2) C. Ilya and Sticks

题目传送门

 /*
     题意：给n个棍子，组成的矩形面积和最大，每根棍子可以-1
     贪心：排序后，相邻的进行比较，若可以读入x[p++]，然后两两相乘相加就可以了
 */
 #include <cstdio>
 #include <algorithm>
 #include <cstring>
 #include <cmath>
 using namespace std;

 typedef long long ll;

 const int MAXN = 1e5 + ;
 const int INF = 0x3f3f3f3f;
 int a[MAXN];
 ll x[MAXN];
 int vis[MAXN];

 int main(void)        //Codeforces Round #297 (Div. 2) C. Ilya and Sticks
 {
     int n;
     while (scanf ("%d", &n) == )
     {
         memset (x, , sizeof (x));
         for (int i=; i<=n; ++i)    scanf ("%d", &a[i]);
         sort (a+, a++n);
         int p = ;
         for (int i=n; i>=; --i)
         {
             if (a[i] == a[i-])    {x[p++] = a[i];    i--;}
             else if (a[i] == a[i-] + )    {x[p++] = a[i-];    i--;}
         }

         if (p < )    puts ("");
         else
         {
             ll sum = ;    x[p] = ;
             for (int i=; i<p; i+=)
             {
                 sum += (x[i] * x[i+]);
             }
             if (p & )    sum -= x[p-];
             printf ("%I64d\n", sum);
         }
     }

     return ;
 }

 /*
 4
 2 4 4 2
 4
 2 2 3 5
 4
 100003 100004 100005 100006
 5
 1 1 1 1 1
 10
 3 3 5 4 2 2 5 6 7 5
 */