LC_4Sum_2.py

"""
454. 4Sum II

Given four lists A, B, C, D of integer values, compute how many tuples (i, j, k, l) there are such that A[i] + B[j] + C[k] + D[l] is zero.

To make problem a bit easier, all A, B, C, D have same length of N where 0 ≤ N ≤ 500. All integers are in the range of -228 to 228 - 1 and the result is guaranteed to be at most 231 - 1.

Example:

Input:
A = [ 1, 2]
B = [-2,-1]
C = [-1, 2]
D = [ 0, 2]

Output:
2

Explanation:
The two tuples are:
1. (0, 0, 0, 1) -> A[0] + B[0] + C[0] + D[1] = 1 + (-2) + (-1) + 2 = 0
2. (1, 1, 0, 0) -> A[1] + B[1] + C[0] + D[0] = 2 + (-1) + (-1) + 0 = 0

Approach:
========
For the first two list, for all  possible sum.. store the-sum into a hashmap with -sum as the key and the count.
for the second two list... if sum is in the hashmap.. add the value of the hasmap to the count.
return count
"""
class Solution(object):
    def fourSumCount(self, A, B, C, D):
        """
        :type A: List[int]
        :type B: List[int]
        :type C: List[int]
        :type D: List[int]
        :rtype: int
        """
        count = 0
        exSum = collections.Counter()
        for a in A:
            for b in B:
                s = a + b
                exSum [-1*s] += 1
        for c in C:
            for d in D:
                s = c + d
                if (s in exSum):
                    count += exSum[s]
        return count