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Cod sursa(job #528170)
#include<stdio.h>
#include<algorithm>
using namespace std;
int x[800],a,i,j,n,st,dr,gasit,mij,g,k,poz;
bool cmp(int a,int b)
{
return a>=b;
}
int main()
{
freopen("nrtri.in","r",stdin);
freopen("nrtri.out","w",stdout);
scanf("%d",&n);
for(i=0;i<n;i++)
{
scanf("%d",&x[i]);
}
sort(x,x+n,cmp);
for(i=0;i<n-2;i++)
{
for(j=i+1;j<n-1;j++)
{
a=x[i]-x[j];
st=j+1;dr=n-1;
poz=-1;
while(st<=dr)
{
mij=(st+dr)/2;
if(x[mij]==a)
{
poz=mij;
st=mij+1;
}
else
{
if(x[mij]>a)
{
st=mij+1;
}
else
{
dr=mij-1;
}
}
}
if(poz>=0)
{
g=g+poz-j;
}
else
{
for(k=j+1;k<n;k++)
if(x[i]<x[j]+x[k])
g++;
else
break;
}
}
}
printf("%d",g);
return 0;
}