solution 1:

sort the array, then fix two, using two pointers, one at the beginning element, one at the end element, moving towards, calculate the sum of 4 elements.

time: O(n^3), space : O(1)

solution 2:

Using a HashMap<Integer, List<Integer>> store the sum, index pair.

Then using 2 for loop iterate the hashMap, find the possible result

time: O(N^2) , space: O(N^2)

http://www.lifeincode.net/programming/leetcode-two-sum-3-sum-3-sum-closest-and-4-sum-java/