leetcode解题之 18. 4Sum Java版（结果是目标值的四个数字和）

18. 4Sum

Given an array S of n integers, are there elementsa, 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.

给一个数组，和一个target整数，求数组中能够使得和为target的所有组合a,b,c,d并满足a<=b<=c<=d

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]]参考：3sum 
public List<List<Integer>> fourSum(int[] nums, int target) {List<List<Integer>> ret = new ArrayList<>();if (nums == null || nums.length < 3)return ret;int len = nums.length;Arrays.sort(nums);// 注意，对于 num[i]，寻找另外两个数时，只要从 i+1 开始找就可以了。// 这种写法，可以避免结果集中有重复，因为数组时排好序的，// 所以当一个数被放到结果集中的时候，其后面和它相等的直接被跳过。for (int i = 0; i < len; i++) {// 避免重复！！！！if (i > 0 && nums[i] == nums[i - 1])continue;for (int j = i + 1; j < len; j++) {// 注意jif (j > i + 1 && nums[j] == nums[j - 1])continue;// 往后找，避免重复int begin = j + 1;int end = len - 1;while (begin < end) {int sum = nums[i] + nums[j] + nums[begin] + nums[end];if (sum == target) {List<Integer> list = new ArrayList<>();list.add(nums[i]);list.add(nums[j]);list.add(nums[begin]);list.add(nums[end]);ret.add(list);begin++;end--;// 避免重复！！！！while (begin < end && nums[begin] == nums[begin - 1])begin++;while (begin < end && nums[end] == nums[end + 1])end--;} else if (sum > target)end--;elsebegin++;}}}return ret;}