add 4sum

Author: sagivo

README.md

@@ -16,6 +16,7 @@ This is my way of saying - change your interview style. There are lots of smart
 | [House Robber](https://leetcode.com/problems/house-robber/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/house-robber.rb) |
 | [Array Product](https://leetcode.com/problems/product-of-array-except-self/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/product-of-array.rb) |
 | [Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/top-k-elements.rb) |
+| [Sum 4 arrays](https://leetcode.com/problems/4sum-ii/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/4-sum.rb) |
 | [Reverse String In-place](https://leetcode.com/problems/reverse-string/description/) | [click](https://github.com/sagivo/algorithms/blob/master/src/reverse-string-inplace.rb) |
 | [Symmetric Tree](https://leetcode.com/problems/symmetric-tree/) | [click](https://github.com/sagivo/algorithms/blob/master/src/mirror.rb) |
 | [Dijkstra's shortest path between two nodes](https://en.wikipedia.org/wiki/Dijkstra%27s_algorithm) | [click](https://github.com/sagivo/algorithms/blob/master/src/dijkstra.rb) |

src/4-sum.rb

@@ -0,0 +1,35 @@
+# @param {Integer[]} a
+# @param {Integer[]} b
+# @param {Integer[]} c
+# @param {Integer[]} d
+# @return {Integer}
+
+def four_sum_count(a, b, c, d)
+ return check_4_sum(a, b, c, d)
+end
+
+def create_set(a, b)
+ h = {}
+ for ai in a
+ for bi in b
+ h[ai + bi] ||= 0
+ h[ai + bi] += 1
+ end
+ end
+ return h
+end
+
+def check_4_sum(a, b, c, d)
+ tot = 0
+ a1 = create_set(a, b)
+ a2 = create_set(c, d)
+ a1.each do |k, v|
+ if a2[-1 * k]
+ tot += v * a2[-1 * k]
+ end
+ end
+ p a1, a2
+ return tot
+end
+
+p four_sum_count([-1,-1], [-1,1], [-1, 1], [1,-1])