4 Values whose Sum is 0（POJ-2785）
Problem 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.
Examples
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 45Output
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：给出 n 行，每行有 4 个数，每一列看做一个组，现在在每个组中选出一个数，问有多少种组合使得选出的 4 个数和为 0
思路：分治法
由于计算 4 个数的和并不好计算，因此可以使用分治的思想，将 4 组数分为两组，然后每组再分别计算和，最后对两组合进行排序，让正数与负数相加判断是否为 0 即可
Source Program
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