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Cod sursa(job #244338)
#include <cstdio>
#include <vector>
#include <queue>
#define fi first
#define se second
using namespace std;
const int NMAX = 1024, HSIZE = 50000;
const double knt = 0.61803398;
int N,S,c[NMAX];
vector< pair<int,int> > hash[HSIZE];
queue < int > Q;
bool inqueue[HSIZE];
inline int h(int x) {
return int( (double(x * knt) - int(x * knt)) * (HSIZE-1) );
}
int search(int x,int ind)
{
int hh = h(x);
for(int i = hash[hh].size() - 1; i >= 0; --i)
if(hash[hh][i] . fi == x) return i;
else
if(hash[hh][i] . fi < 0 && hash[hh][i] . fi > -ind) break;
return -1;
}
void insert(int x,int ind) {
int hh = h(x), t = search(x,ind);
if(t == -1)
hash[hh].push_back(make_pair(x,1));
else hash[hh][t] . se ++;
if(!inqueue[hh]) Q.push(hh), inqueue[hh] = 1;
}
int count(int x,int ind) {
if(x<0) return 0;
int cnt = 0,i,pos = h(x);
for(i = hash[pos].size() - 1; i >= 0; --i)
{
if(hash[pos][i] . fi == x) cnt += hash[pos][i] . se;
else
if(hash[pos][i] . fi < 0 && hash[pos][i] . fi > -ind) break;
}
return cnt;
}
/* explicatie hash-ul retine si primul indice i al sumelor (i,j)
cu valoare negativa ( -i) si daca gasim elemente egale cu x
cu indicele negativ mai mare (adica cu indicele pozitiv mai mic)
decat i cnt devine 0
*/
void debug()
{
for(int i=0;i < HSIZE; ++i)
{
for(int j=0; j < (int)hash[i].size(); ++j)
printf("(%d %d) ",hash[i][j]. fi, hash[i][j] . se);
printf("\n");
}
}
int main() {
freopen("oite.in","r",stdin);
freopen("oite.out","w",stdout);
scanf("%d%d",&N,&S);
for(int i=1;i<=N;++i)
scanf("%d",&c[i]);
for(int i=1;i<N;++i)
{
for(int j=i+1;j<=N;++j)
insert(c[i]+c[j],i);
while(!Q.empty())
{
hash[Q.front()].push_back( make_pair(-i,-i) );
Q.pop();
}
}
int cnt = 0;
for(int i=1;i<N;++i)
for(int j=i+1;j<=N;++j)
cnt += count(S - c[i] - c[j],j+1);
printf("%d",cnt);
return 0;
}