poj 3663 Costume Party(快排)

#include<stdio.h>
#include<iostream>
#include<cstring>
#include<algorithm>
using namespace std;
int  num[20010];
int main(){
int n,m,i,j,t,v;
while(~scanf("%d%d",&n,&m)){
memset(num,0,sizeof(num));
for(i=0;i<n;i++){
scanf("%d",&num[i]);
}
sort(num,num+n);
for(t=i=0;i<n;i++){
for(j=i+1;j<n;j++)
if(num[i]+num[j]<=m){
t++;
}else{
break;
}
}
printf("%d\n",t);
}
return 0;
}

//运行时间超短的，不容易懂
#include<cstdio>
#include<cstring>
#include<iostream>
#include<algorithm>
using namespace std;
int num[1000010];
int main(){
int n,m,i,s,l;
while(~scanf("%d%d",&n,&m)){
memset(num,0,sizeof(num));
for(i=0;i<n;i++){
scanf("%d",&num[i]);
}
sort(num,num+n);//快排
for(s=l=0,i=n-1;i>=0;i--){
while(num[l]+num[i]<=m&&l<n){//找到不满足和小于m的位置
l++;
}
s+=l;
if(l-1>=i) s--;
}
printf("%d\n",s/2);
}
return 0;
}