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)

public class Solution {
    public List<List<Integer>> fourSum(int[] nums, int target) {
    	List<List<Integer>> list = new ArrayList<List<Integer>>();  
        Arrays.sort(nums);  
        Set<ArrayList<Integer>> set = new HashSet<ArrayList<Integer>>();  
        for (int i = 0; i < nums.length-3; i++) {  
            int a = nums[i];  
            for (int j = i+1; j < nums.length-2; j++) {
            	int d = nums[j];
            	int start = j+1;  
                int end = nums.length-1;  
                while (start < end) {  
                    int b = nums[start];  
                    int c = nums[end];  
                    if (a+b+c+d == target) {  
                        ArrayList<Integer> t = new ArrayList<Integer>();  
                        t.add(a);  
                        t.add(d);
                        t.add(b);  
                        t.add(c); 
                        set.add(t);  
                        start++;  
                        end--;  
                    } else if (a+b+c+d < target) {  
                        start++;  
                    } else {  
                        end--;  
                    }  
                }  
            }
        }  
        list.addAll(set);  
        return list;  
    }
}