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] ]
此题和three sum比较类似，求法也差不多，直接上代码：
public class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> res = new ArrayList<>();
Arrays.sort(nums);
for(int i=0;i<nums.length-3;i++){
if(i!=0&&nums[i]==nums[i-1]) continue;
for(int j=i+1;j<nums.length-2;j++){
if(j!=i+1&&nums[j]==nums[j-1]) continue;
int low = j+1,high = nums.length-1;
while(low<high){
int sum = nums[i]+nums[j]+nums[low]+nums[high];
if(sum<target){
low++;
}else if(sum>target){
high--;
}else{
res.add(Arrays.asList(nums[i],nums[j],nums[low],nums[high]));
while(low<high&&nums[low]==nums[low+1]) low++;
while(low<high&&nums[high]==nums[high-1]) high--;
low++;
high--;
}
}
}
}
return res;
}
}