Java solution 18

Author: gavinfish

java/_0184Sum.java

@@ -0,0 +1,70 @@
+import java.util.*;
+
+/**
+ * Given an array S of n integers, are there elements a, b, c, and d in S such that a + b + c + d = target? Find all unique quadruplets in the array which gives the sum of target.
+ * <p>
+ * Note: The solution set must not contain duplicate quadruplets.
+ * <p>
+ * For example, given array S = [1, 0, -1, 0, -2, 2], and target = 0.
+ * A solution set is:
+ * [
+ *  [-1,  0, 0, 1],
+ *  [-2, -1, 1, 2],
+ *  [-2,  0, 0, 2]
+ * ]
+ * <p>
+ * Created by drfish on 6/20/2017.
+ */
+public class _0184Sum {
+    public List<List<Integer>> fourSum(int[] nums, int target) {
+        List<List<Integer>> result = new ArrayList<>();
+        if (nums.length < 4) {
+            return result;
+        }
+        Arrays.sort(nums);
+        for (int i = 0; i < nums.length - 3; i++) {
+            if (nums[i] + nums[i + 1] + nums[i + 2] + nums[i + 3] > target) {
+                break;
+            }
+            if (nums[i] + nums[nums.length - 1] + nums[nums.length - 2] + nums[nums.length - 3] < target) {
+                continue;
+            }
+            if (i > 0 && nums[i] == nums[i - 1]) {
+                continue;
+            }
+
+            for (int j = i + 1; j < nums.length - 2; j++) {
+                if (nums[i] + nums[j] + nums[j + 1] + nums[j + 2] > target) {
+                    break;
+                }
+                if (nums[i] + nums[j] + nums[nums.length - 1] + nums[nums.length - 2] < target) {
+                    continue;
+                }
+                if (j > i + 1 && nums[j] == nums[j - 1]) {
+                    continue;
+                }
+
+                int low = j + 1, high = nums.length - 1;
+                while (low < high) {
+                    int sum = nums[i] + nums[j] + nums[low] + nums[high];
+                    if (sum == target) {
+                        result.add(Arrays.asList(nums[i], nums[j], nums[low], nums[high]));
+                        while (low < high && nums[low] == nums[low + 1]) {
+                            low++;
+                        }
+                        while (low < high && nums[high] == nums[high - 1]) {
+                            high--;
+                        }
+                        low++;
+                        high--;
+                    } else if (sum < target) {
+                        low++;
+                    } else {
+                        high--;
+                    }
+                }
+            }
+        }
+        return result;
+    }
+}