题目链接:AtCoder Beginner Contest 143 B-TAKOYAKI FESTIVAL 2019
题解
模拟即可
#include<cstdio>
#include<algorithm>
using std::max;
#define reg register
#define getchar() (p1==p2&&(p2=(p1=buf)+fread(buf,1,100000,stdin),p1==p2)?EOF:*p1++)
static char buf[100000],*p1=buf,*p2=buf;
inline int read(void){
reg bool f=false;
reg char ch=getchar();
reg int res=0;
while(ch<'0'||'9'<ch)f|=(ch=='-'),ch=getchar();
while('0'<=ch&&ch<='9')res=10*res+ch-'0',ch=getchar();
return f?-res:res;
}
const int MAXN=50+5;
int n;
int d[MAXN];
int main(void){
reg int i,j,ans=0;
n=read();
for(i=1;i<=n;++i)
d[i]=read();
for(i=1;i<=n;++i)
for(j=i+1;j<=n;++j)
ans+=d[i]*d[j];
printf("%d\n",ans);
return 0;
}
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