Update

problems/0018.四数之和.md

@@ -244,56 +244,61 @@ int** fourSum(int* nums, int numsSize, int target, int* returnSize, int** return
 ### Java：
 
 ```Java
-class Solution {
+import java.util.*;
+
+public 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++) {
-		
-            // nums[i] > target 直接返回, 剪枝操作
-            if (nums[i] > 0 && nums[i] > target) {
-                return result;
+        Arrays.sort(nums);  // 排序数组
+        List<List<Integer>> result = new ArrayList<>();  // 结果集
+        for (int k = 0; k < nums.length; k++) {
+            // 剪枝处理
+            if (nums[k] > target && nums[k] >= 0) {
+                break;
             }
-		
-            if (i > 0 && nums[i - 1] == nums[i]) {    // 对nums[i]去重
+            // 对nums[k]去重
+            if (k > 0 && nums[k] == nums[k - 1]) {
                 continue;
             }
-            
-            for (int j = i + 1; j < nums.length; j++) {
-
-		// nums[i]+nums[j] > target 直接返回, 剪枝操作
-		            if (nums[i]+nums[j] > 0 && nums[i]+nums[j] > target) {
-		                return result;
-		            }
-
-                if (j > i + 1 && nums[j - 1] == nums[j]) {  // 对nums[j]去重
+            for (int i = k + 1; i < nums.length; i++) {
+                // 第二级剪枝
+                if (nums[k] + nums[i] > target && nums[k] + nums[i] >= 0) {
+                    break;
+                }
+                // 对nums[i]去重
+                if (i > k + 1 && nums[i] == nums[i - 1]) {
                     continue;
                 }
-
-                int left = j + 1;
+                int left = i + 1;
                 int right = nums.length - 1;
                 while (right > left) {
-		    // nums[k] + nums[i] + nums[left] + nums[right] > target int会溢出
-                    long sum = (long) nums[i] + nums[j] + nums[left] + nums[right];
+                    long sum = (long) nums[k] + nums[i] + 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]));
+                        result.add(Arrays.asList(nums[k], nums[i], nums[left], nums[right]));
                         // 对nums[left]和nums[right]去重
                         while (right > left && nums[right] == nums[right - 1]) right--;
                         while (right > left && nums[left] == nums[left + 1]) left++;
-
-                        left++;
                         right--;
+                        left++;
                     }
                 }
             }
         }
         return result;
     }
+
+    public static void main(String[] args) {
+        Solution solution = new Solution();
+        int[] nums = {1, 0, -1, 0, -2, 2};
+        int target = 0;
+        List<List<Integer>> results = solution.fourSum(nums, target);
+        for (List<Integer> result : results) {
+            System.out.println(result);
+        }
+    }
 }
 ```