Merge pull request #18 from Powerstot/patch-2

添加0018.四数之和 Java版本

problems/0018.四数之和.md

@@ -121,7 +121,48 @@ public:
 
 
 Java：
+```Java
+class Solution {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        Arrays.sort(nums);
        
+        for (int i = 0; i < nums.length; i++) {
+
+            if (i > 0 && nums[i - 1] == nums[i]) {
+                continue;
+            }
+            
+            for (int j = i + 1; j < nums.length; j++) {
+
+                if (j > i + 1 && nums[j - 1] == nums[j]) {
+                    continue;
+                }
+
+                int left = j + 1;
+                int right = nums.length - 1;
+                while (right > left) {
+                    int sum = nums[i] + nums[j] + nums[left] + nums[right];
+                    if (sum > target) {
+                        right--;
+                    } else if (sum < target) {
+                        left++;
+                    } else {
+                        result.add(Arrays.asList(nums[i], nums[j], nums[left], nums[right]));
+                        
+                        while (right > left && nums[right] == nums[right - 1]) right--;
+                        while (right > left && nums[left] == nums[left + 1]) left++;
+
+                        left++;
+                        right--;
+                    }
+                }
+            }
+        }
+        return result;
+    }
+}
+```
 
 Python：