题目描述：给定一个包含n 个整数的数组nums和一个目标值target，判断nums中是否存在四个元素 a，b，c和 d，使得a + b + c + d的值与target相等？找出所有满足条件且不重复的四元组。
示例：给定数组 nums = [1, 0, -1, 0, -2, 2]，和 target = 0。满足要求的四元组集合为：[ [-1, 0, 0, 1], [-2, -1, 1, 2], [-2, 0, 0, 2] ]
java代码：
class Solution {
public List<List<Integer>> fourSum(int[] nums, int target) {
List<List<Integer>> res = new LinkedList<>();
Arrays.sort(nums);
int n = nums.length;
for(int i=0;i<n-3;i++) {
if(i>0 && nums[i] == nums[i-1]) continue;
if(nums[i]+nums[i+1]+nums[i+2]+nums[i+3]>target) break;
if(nums[i]+nums[n-1]+nums[n-2]+nums[n-3]<target) continue;
for(int j=i+1;j<n-2;j++) {
if(j-i>1 && nums[j]==nums[j-1]) continue;
if(nums[i]+nums[j]+nums[j+1]+nums[j+2]>target) break;
if(nums[i]+nums[j]+nums[n-1]+nums[n-2]<target) continue;
int left = j + 1;
int right = n - 1;
while(left < right) {
int tmp = nums[i] + nums[j] + nums[left] + nums[right];
if(tmp == target) {
List<Integer> tmpList = new LinkedList<>(Arrays.asList(nums[i],nums[j],nums[left],nums[right]));
res.add(tmpList);
while(left < right && nums[left] == nums[left+1]) left += 1;
while(left < right && nums[right] == nums[right-1]) right -= 1;
left += 1;
right -= 1;
}else if(tmp > target) right -= 1;
else left += 1;
}
}
}
return res;
}
}
