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]
]

分析：
從數組中找出四個數字之和等于目標值。
類似3Sum铁追。http://www.reibang.com/p/4bc0920417bb

Java語言：

    public List<List<Integer>> fourSum(int[] nums, int target) {
        Arrays.sort(nums);
        int len = nums.length;
        List<List<Integer>> res = new ArrayList<>();
        for (int i = 0; i < len - 3; i++) {
            if (i > 0 && nums[i] == nums[i - 1]) {//防止重復
                continue;
            }
            for (int j = i + 1; j < len - 2; j++) {
                if (j > i + 1 && nums[j] == nums[j - 1]) {//防止重復
                    continue;
                }
                int lo = j + 1, hi = len - 1;
                while (lo < hi) {
                    int sum = nums[i] + nums[j] + nums[lo] + nums[hi];
                    if (sum == target) {
                        res.add(Arrays.asList(nums[i], nums[j], nums[lo], nums[hi]));
                        while (lo < hi && nums[lo] == nums[lo + 1]) {//防止重復
                            lo++;
                        }
                        while (lo < hi && nums[hi] == nums[hi - 1]) {
                            hi--;
                        }
                        lo++;
                        hi--;
                    } else if (sum < target) {
                        lo++;
                    } else {
                        hi--;
                    }
                }
            }
        }
        return res;
    }