POJ 2785：4 Values whose Sum is 0 （双向BFS）

POJ 2785：4 Values whose Sum is 0 （双向BFS）

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 2 28 ) 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).

题意：求在四组数中各挑选一个满足a+b+c+d==0的个数。

直接使用BFS时间复杂度是O（n^4），显然会超时，我们可以把整个四组数据分成两组，然后就是对两组数据分别BFS，最终统计结果，这样便把时间复杂度降低到了O（n^2），很容易就可以过啦~

AC代码：

#include#include#include#include#define max(a,b) (a>b?a:b)using namespace std;__int64 a[4005],b[4005],c[4005],d[4005];__int64 cd[4005*4005];int main(){ int n; scanf("%d",&n); for(int i=0; i

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