题目传送门
1 /*
2 题意：给n个棍子，组成的矩形面积和最大，每根棍子可以-1
3 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
4 */
5 #include <cstdio>
6 #include <algorithm>
7 #include <cstring>
8 #include <cmath>
9 using namespace std;
10
11 typedef long long ll;
12
13 const int MAXN = 1e5 + 10;
14 const int INF = 0x3f3f3f3f;
15 int a[MAXN];
16 ll x[MAXN];
17 int vis[MAXN];
18
19 int main(void) //Codeforces Round #297 (Div. 2) C. Ilya and Sticks
20 {
21 int n;
22 while (scanf ("%d", &n) == 1)
23 {
24 memset (x, 0, sizeof (x));
25 for (int i=1; i<=n; ++i) scanf ("%d", &a[i]);
26 sort (a+1, a+1+n);
27 int p = 0;
28 for (int i=n; i>=2; --i)
29 {
30 if (a[i] == a[i-1]) {x[p++] = a[i]; i--;}
31 else if (a[i] == a[i-1] + 1) {x[p++] = a[i-1]; i--;}
32 }
33
34 if (p < 2) puts ("0");
35 else
36 {
37 ll sum = 0; x[p] = 1;
38 for (int i=0; i<p; i+=2)
39 {
40 sum += (x[i] * x[i+1]);
41 }
42 if (p & 1) sum -= x[p-1];
43 printf ("%I64d
", sum);
44 }
45 }
46
47 return 0;
48 }
49
50
51 /*
52 4
53 2 4 4 2
54 4
55 2 2 3 5
56 4
57 100003 100004 100005 100006
58 5
59 1 1 1 1 1
60 10
61 3 3 5 4 2 2 5 6 7 5
62 */