Question Definition
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: 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]
]
Java Solution
public List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
List<List<Integer>> result = new LinkedList<>();
for(int j = 0; j < nums.length - 3; j++){
if (j > 0 && nums[j] == nums[j - 1]) continue;
for(int i = j + 1; i < nums.length - 2; i++){
if (i > j + 1 && nums[i] == nums[i - 1]) continue;
int left = i + 1, right = nums.length - 1;
while(left < right){
if(nums[i] + nums[j] + nums[left] + nums[right] == target){
List<Integer> cur = new LinkedList<>();
cur.add(nums[j]);
cur.add(nums[i]);
cur.add(nums[left]);
cur.add(nums[right]);
result.add(cur);
while (left < right && nums[left] == nums[left + 1]) ++left;
while (left < right && nums[right] == nums[right - 1]) --right;
left++;
right--;
}else if(nums[i] + nums[j] + nums[left] + nums[right] < target)
left++;
else
right--;
}
}
}
return result;
}
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