添加0018.四数之和 Ruby实现

problems/0018.四数之和.md

@@ -649,6 +649,54 @@ object Solution {
   }
 }
 ```
+### Ruby:
+
+```ruby
+def four_sum(nums, target)
+  #结果集
+  result = []
+  nums = nums.sort!
+
+  for i in 0..nums.size - 1
+    return result if i > 0 && nums[i] > target && nums[i] >= 0
+    #对a进行去重
+    next if i > 0 && nums[i] == nums[i - 1]
+
+    for j in i + 1..nums.size - 1
+      break if nums[i] + nums[j] > target && nums[i] + nums[j] >= 0
+      #对b进行去重
+      next if j > i + 1 && nums[j] == nums[j - 1]
+      left = j + 1
+      right = nums.size - 1
+      while left < right
+        sum = nums[i] + nums[j] + nums[left] + nums[right]
+        if sum > target
+          right -= 1
+        elsif sum < target
+          left += 1
+        else
+          result << [nums[i], nums[j], nums[left], nums[right]]
+
+          #对c进行去重
+          while left < right && nums[left] == nums[left + 1]
+            left += 1
+          end
+
+          #对d进行去重
+          while left < right && nums[right] == nums[right - 1]
+            right -= 1
+          end
+
+          right -= 1
+          left += 1
+        end
+      end
+    end
+  end
+
+  return result
+end
+```
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