4Sum

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)
在3Sum的基础上稍微修改即可，O(n ^ 3)。

 1 public class Solution {

 2     public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {

 3         // Note: The Solution object is instantiated only once and is reused by each test case.

 4          Arrays.sort(num);

 5         ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();

 6         if(num == null || num.length < 4) return result;

 7         int len = num.length;

 8         for(int i = 0; i <= num.length - 4; i ++){

 9             if(i==0 || num[i] > num[i-1]){

10                 for(int a = num.length - 1; a > i + 2; a --){

11                     if(a == num.length - 1 || num[a] < num[a + 1]){

12                         int j = i + 1;

13                         int h = a - 1;

14                         while(j < h){

15                             int sum = num[j] + num[h] + num[i] + num[a];

16                             if(sum == target){

17                                 ArrayList<Integer> row = new ArrayList<Integer>();

18                                 row.add(num[i]);

19                                 row.add(num[j]);

20                                 row.add(num[h]);

21                                 row.add(num[a]);

22                                 result.add(row);

23                                 h--;

24                                 j++;

25                                 while(h>j && num[h]==num[h+1]) h--; 

26         

27                                 while(j<h && num[j]==num[j-1]) j++;

28                             }else if(sum < target){

29                                     j ++;

30                             }else if(sum > target){

31                                     h --;

32                             }

33                         }

34                     }

35                 }

36             }

37         }

38         return result;

39     }

40 }