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);
LinkedList<Integer> cur = new LinkedList<>();
List<List<Integer>> result = new LinkedList<>();
fourSumHelper(nums, cur, 4, target, 0, nums.length - 1, result);
return result;
}
private void fourSumHelper(int[] nums, LinkedList<Integer> cur, int k, int target, int start, int end, List<List<Integer>> result){
if(k <= 0)
return;
if(k == 1){
for(int i = start; i <= end; i++){
if(nums[i] == target){
LinkedList<Integer> temp = new LinkedList<>(cur);
temp.add(nums[i]);
result.add(temp);
}
}
return;
}
if(k == 2){
twoSumHelper(nums, cur, k, target, start, end, result);
return;
}
for(int i = start; i <= end - k + 1; i++) {
if(i > start && nums[i] == nums[i-1]) continue;
cur.addLast(nums[i]);
fourSumHelper(nums, cur, k - 1, target - nums[i], i + 1, end, result);
cur.removeLast();
}
return;
}
private void twoSumHelper(int[] nums, LinkedList<Integer> cur, int k, int target, int start, int end, List<List<Integer>> result){
while(start < end){
int sum = nums[start] + nums[end];
if(sum == target) {
LinkedList<Integer> temp = new LinkedList<>(cur);
temp.add(nums[start]);
temp.add(nums[end]);
result.add(temp);
start++;
end--;
while(start < nums.length && nums[start] == nums[start-1]) start++;
while(end >= 0 && nums[end] == nums[end+1]) end--;
}
else if(sum < target)
start++;
else
end--;
}
return;
}
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