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 ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
// Start typing your Java solution below
// DO NOT write main() function
ArrayList<ArrayList<Integer>> output = new ArrayList<ArrayList<Integer>>();
HashSet<ArrayList<Integer>> resultSet = new HashSet<ArrayList<Integer>>();
if(num.length<4){
return output;
}
Arrays.sort(num);
for(int i=0;i<num.length-3;i++){
for(int j=i+1;j<num.length-2;j++){
int k = j+1;
int l = num.length-1;
while(k<l){
int sum = num[i]+num[j]+num[k]+num[l];
if(sum==target){
ArrayList<Integer> result = new ArrayList<Integer>();
result.add(num[i]);
result.add(num[j]);
result.add(num[k]);
result.add(num[l]);
if(resultSet.add(result)){
output.add(result);
}
k++;
l--;
}else if(sum<target){
k++;
}else{
l--;
}
}
}
}
return output;
}
O(N^3) time and O(n^3) space needed. Waiting for more efficient solutions.