LeetCode_OJ【18】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:

• 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)

此题思路和 LeetCode_OJ【15】3Sum 以及 LeetCode_OJ【16】3Sum Closest 思路类似，这里不再赘述，实现如下：

public class Solution {    public List<List<Integer>> fourSum(int[] nums, int target) {        Arrays.sort(nums);List<List<Integer>> list = new ArrayList<List<Integer>>();for(int i = 0 ; i < nums.length - 3 ; i++){if(i != 0 && nums[i-1] == nums[i])continue;for(int j = i + 1; j < nums.length -2 ; j++){if(j != i+1 && nums[j-1] == nums[j])continue;for(int left = j + 1,right = nums.length -1; left < right ;){if(nums[i] + nums[j] + nums[left] + nums[right] == target){if(left != j + 1 && nums[left-1] == nums[left]){left ++;continue;}List<Integer> tmp = new ArrayList<Integer>();tmp.add(nums[i]);tmp.add(nums[j]);tmp.add(nums[left]);tmp.add(nums[right]);list.add(tmp);left ++;}else if(nums[i] + nums[j] + nums[left] + nums[right] > target){right --;}else {left ++ ;}}}}        return list;    }}