POJ 2785 4 Values whose Sum is 0

Description

The SUM problem can be formulated as follows: given four lists A, B, C, D of integer values, compute how many quadruplet (a, b, c, d ) ∈ A x B x C x D are such that a + b + c + d = 0 . In the following, we assume that all lists have the same size n .

Input

The first line of the input file contains the size of the lists n (this value can be as large as 4000). We then have n lines containing four integer values (with absolute value as large as 2²⁸ ) that belong respectively to A, B, C and D .

Output

For each input file, your program has to write the number quadruplets whose sum is zero.

Sample Input

6

-45 22 42 -16

-41 -27 56 30

-36 53 -37 77

-36 30 -75 -46

26 -38 -10 62

-32 -54 -6 45

Sample Output

5

Hint

 

Sample Explanation: Indeed, the sum of the five following quadruplets is zero: (-45, -27, 42, 30), (26, 30, -10, -46), (-32, 22, 56, -46),(-32, 30, -75, 77), (-32, -54, 56, 30).

【代码】：

【Time】：O（N²logN）

#include <iostream>
#include <algorithm>
#include<cstdio>
using namespace std;
#define N 4005

int n, a[N], b[N], c[N], d[N], cd[N*N];
int main()
{
    cin >> n;
     for(int i = 0;i < n; i++)
        scanf("%d%d%d%d",&a[i],&b[i],&c[i],&d[i]);

     for(int i = 0;i < n; i++){
         for(int j = 0;j < n; j++){
            cd[i*n+j] = c[i] + d[j];
        }
    }
    sort(cd,cd+n*n);
    long long ans = 0;
     for(int i = 0;i < n; i++){
         for(int j = 0;j < n; j++){
            int x = -(a[i] + b[j]);
            ans += upper_bound(cd, cd+n*n, x) - lower_bound(cd, cd+n*n, x);
        }
    }
    cout << ans << endl;
    return 0;
}
/*
6
-45 -41 -36 -36 26 -32
22 -27 53 30 -38 -54
42 56 -37 -75 -10 -6
-16 30 77 -46 62 45
*/