Given an array nums of n integers and an integer target, are there elements a, b, c, and d in nums 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.
Example:
Given array nums = [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 List<List<Integer>> fourSum(int[] nums, int target) {
Arrays.sort(nums);
List<List<Integer>> res = new ArrayList<>();
for(int i = 0; i < nums.length - 3; i++){
for(int j = i+1; j < nums.length - 2; j++){
int sum = target - nums[i] - nums[j];
int front = j + 1;
int back = nums.length - 1;
while(front < back){
if(nums[front] + nums[back] == sum){
res.add(Arrays.asList(nums[i],nums[j],nums[front],nums[back]));
while(front < back && nums[front+1] == nums[front]) front++;
while(front < back && nums[back-1] == nums[back]) back--;
front++;
back--;
}
else if(nums[front] + nums[back] < sum){
front++;
}else{
back--;
}
}
while(j+1 < nums.length-2 && nums[j+1] == nums[j]) j++;
}
while(i+1 < nums.length-3 && nums[i+1] == nums[i]) i++;
}
return res;
}