Java [leetcode 18]4Sum

標籤：

問題描述：

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:

• Elements in a quadruplet (a,b,c,d) must be in non-descending order. (ie, a ≤ b ≤ c ≤ d)
• 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)

解題思路：

與之前的3sum相類似，只不過在外邊多了一套迴圈，時間複雜度為O(n^3)

代碼如下：

public class Solution {    List<List<Integer>> ans = new ArrayList<List<Integer>>();public List<List<Integer>> fourSum(int[] nums, int target) {int length = nums.length;int tmp;if (nums == null || length < 4)return ans;Arrays.sort(nums);for (int i = 0; i < length - 3; i++) {if (i > 0 && nums[i] == nums[i - 1])continue;for (int j = i + 1; j < length - 2; j++) {if (j > i + 1 && nums[j] == nums[j - 1])continue;int begin = j + 1;int end = length - 1;while (begin < end) {tmp = nums[begin] + nums[end] + nums[i] + nums[j];if (tmp == target) {List<Integer> list = new ArrayList<Integer>();list.add(nums[i]);list.add(nums[j]);list.add(nums[begin]);list.add(nums[end]);ans.add(list);while (begin < end && nums[begin + 1] == nums[begin])begin++;begin++;while (begin < end && nums[end - 1] == nums[end])end--;end--;} else if (tmp > target)end--;elsebegin++;}}}return ans;}}

 

Java [leetcode 18]4Sum