Equations-hdu1946-二分查找

Consider equations having the following form: 

a*x1^2+b*x2^2+c*x3^2+d*x4^2=0 
a, b, c, d are integers from the interval [-50,50] and any of them cannot be 0. 

It is consider a solution a system ( x1,x2,x3,x4 ) that verifies the equation, xi is an integer from [-100,100] and xi != 0, any i ∈{1,2,3,4}. 

Determine how many solutions satisfy the given equation. 

InputThe input consists of several test cases. Each test case consists of a single line containing the 4 coefficients a, b, c, d, separated by one or more blanks. 
End of file.OutputFor each test case, output a single line containing the number of the solutions. 
Sample Input

1 2 3 -4
1 1 1 1

Sample Output

39088
0
题意：给出四个整数：a、b、c、d，问满足四元二次方程a*x1^2+b*x2^2+c*x3^2+d*x4^2=0的解有多少个？

#include <stdio.h>
#include <string.h>
#include <iostream>
#include <algorithm>
using namespace std;
typedef long long LL;
LL sum[400007];
LL ans,k;
void erfen(LL t)
{
    int l=0,r=k;
    int mid;
    while(l<=r)
    {
        mid=(l+r)/2;
        if(sum[mid]==t)
        {
            ans++;
            for(int x=mid-1; x>=1&&sum[x]==t; x--)//如果mid左方的位置还有与t相等的数，ans+1
            {
                if(sum[x]==t)
                    ans++;
            }
            for(int x=mid+1; x<k&&sum[x]==t; x++)//如果mid右方的位置还有与t相等的数，ans+1
            {
                if(sum[x]==t)
                    ans++;
            }
            return;
        }
        else if(sum[mid]<t)
            l=mid+1;
        else
            r=mid-1;
    }
}
int main()
{
    int a,b,c,d;
    while(~scanf("%d%d%d%d",&a,&b,&c,&d))
    {
        k=0;
        if(a>0&&b>0&&c>0&&d>0)//剪枝
        {
            printf("0\n");
            continue;
        }
        for(int i=1; i<=100; i++)
            for(int j=1; j<=100; j++)
                sum[k++]=a*i*i+b*j*j;
        sort(sum,sum+k);
        ans=0;
        for(int i=1; i<=100; i++)
            for(int j=1; j<=100; j++)
            {
                LL t=-(c*i*i+d*j*j);
                erfen(t);
            }
        printf("%I64d\n",ans*16);
    }
    return 0;
}