Add Solution2.java to problems 18

Author: huaxu1024

solution/0018.4Sum/Solution2.java

@@ -0,0 +1,30 @@
+class Solution {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        if (nums.length < 4) {
+            return new ArrayList<>();
+        }
+        Arrays.sort(nums);
+
+        List<List<Integer>> results = new ArrayList<>();
+
+        for (int i = 0; i < nums.length; i++) {
+            for (int j = i + 1; j < nums.length; j++) {
+                int low = j + 1, high = nums.length - 1;
+                int sub = target - nums[i] - nums[j];
+                while (low < high) {
+                    if (nums[low] + nums[high] < sub) {
+                        low++;
+                    } else if (nums[low] + nums[high] > sub) {
+                        high--;
+                    } else {
+                        results.add(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
+                        low++;
+                        high--;
+                    }
+                }
+            }
+        }
+
+        return results.stream().distinct().collect(Collectors.toList());
+    }
+}