4 Sum leetcode java

题目：

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.

Note:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• The solution set must not contain duplicate quadruplets.

    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)

题解：
4 sum跟3 sum是一样的思路，只不过需要多考虑一个加数，这样时间复杂度变为O(n3)。

使用HashSet来解决重复问题的代码如下：

 1 public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
 2     HashSet<ArrayList<Integer>> hashSet = new HashSet<ArrayList<Integer>>();
 3     ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
 4     Arrays.sort(num);
 5     for (int i = 0; i <= num.length-4; i++) {
 6         for (int j = i + 1; j <= num.length-3; j++) {
 7             int low = j + 1;
 8             int high = num.length - 1;
 9  
             while (low < high) {
                 int sum = num[i] + num[j] + num[low] + num[high];
  
                 if (sum > target) {
                     high--;
                 } else if (sum < target) {
                     low++;
                 } else if (sum == target) {
                     ArrayList<Integer> temp = new ArrayList<Integer>();
                     temp.add(num[i]);
                     temp.add(num[j]);
                     temp.add(num[low]);
                     temp.add(num[high]);
  
                     if (!hashSet.contains(temp)) {
                         hashSet.add(temp);
                         result.add(temp);
                     }
  
                     low++;
                     high--;
                 }
             }
         }
     }
  
     return result;
 }

使用挪动指针的方法来解决重复的代码如下：

 1 public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
 2     HashSet<ArrayList<Integer>> hashSet = new HashSet<ArrayList<Integer>>();
 3     ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
 4     Arrays.sort(num);
 5     for (int i = 0; i <= num.length-4; i++) {
 6         if(i==0||num[i]!=num[i-1]){
 7             for (int j = i + 1; j <= num.length-3; j++) {
 8                 if(j==i+1||num[j]!=num[j-1]){
 9                     int low = j + 1;
                     int high = num.length - 1;
          
                     while (low < high) {
                         int sum = num[i] + num[j] + num[low] + num[high];
          
                         if (sum > target) {
                             high--;
                         } else if (sum < target) {
                             low++;
                         } else if (sum == target) {
                             ArrayList<Integer> temp = new ArrayList<Integer>();
                             temp.add(num[i]);
                             temp.add(num[j]);
                             temp.add(num[low]);
                             temp.add(num[high]);
          
                             if (!hashSet.contains(temp)) {
                                 hashSet.add(temp);
                                 result.add(temp);
                             }
          
                             low++;
                             high--;
                             
                             while(low<high&&num[low]==num[low-1])//remove dupicate
                                 low++;
                             while(low<high&&num[high]==num[high+1])//remove dupicate
                                 high--;
                         }
                     }
                 }
             }
         }
     }
  
     return result;
 }