leetcode

4Sum

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]]

Solution:

  public List<List<Integer>> fourSum(int[] nums, int target) {        List<List<Integer>> result = new ArrayList<>();        if (nums == null || nums.length < 4) {            return result;        }        Arrays.sort(nums);        int max = nums[nums.length - 1];        if (4 * max < target || 4 * nums[0] > target) {            return result;        }        for (int i = 0; i < nums.length; i++) {            int x = nums[i];            if (i > 0 && x == nums[i - 1]) {                continue;            } else if (x + 3 * max < target) {                continue;            } else if (x + 3 * nums[0] > target) {                break;            } else if (x * 4 == target) {                if (i + 3 < nums.length && nums[i + 3] == x) {                    result.add(Arrays.asList(x, x, x, x));                }            } else {                threeSum(nums, target - x, i + 1, result, x);            }        }        return result;    }    private void threeSum(int[] nums, int target, int begin, List<List<Integer>> result, int x1) {        if (begin + 2 >= nums.length) {            return;        }        int max = nums[nums.length - 1];        if (3 * nums[begin] > target) {            return;        }        for (int i = begin; i < nums.length; i++) {            int x = nums[i];            if (i > begin && x == nums[i - 1]) {                continue;            } else if (x + 2 * max < target) {                continue;            } else if (x + 2 * nums[begin] > target) {                break;            } else if (x * 3 == target) {                if (i + 2 < nums.length && nums[i + 2] == x) {                    result.add(Arrays.asList(x1, x, x, x));                }            } else {                twoSum(nums, target - x, i + 1, result, x1, x);            }        }    }    private void twoSum(int[] nums, int target, int begin, List<List<Integer>> result, int x1, int x2) {        if (begin + 1 >= nums.length) {            return;        }        if (2 * nums[begin] > target) {            return;        }        int i = begin, j = nums.length - 1, sum, k;        while (i < j) {            sum = nums[i] + nums[j];            if (sum == target) {                result.add(Arrays.asList(x1, x2, nums[i], nums[j]));                k = nums[i];                while (++i < j && k == nums[i])                    ;                k = nums[j];                while (i < --j && k == nums[j])                    ;            } else if (sum > target) {                j--;            } else {                i++;            }        }        return;    }