LeetCode 135 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)

分析：

N sum问题比较特殊，规定了数目，因此用回溯解决不合适。

试不同的组合，将满足题意的加入结果集。

public class Solution {
    public List<List<Integer>> fourSum(int[] num, int target) {
        List<List<Integer>> res = new ArrayList<List<Integer>>();
        if(num == null || num.length < 4) return res;
        //HashSet来防止重复
        HashSet<List<Integer>> hashSet = new HashSet<List<Integer>>();
        Arrays.sort(num);
        //尝试所有可能组合，过程中保证i<j<k<l即可。
        for(int i=0; i<num.length-3; i++)
            for(int j=i+1; j<num.length-2; j++){
                int k=j+1;
                int l=num.length-1;
                while(k<l){
                    int sum = num[i]+num[j]+num[k]+num[l];
                    if(sum < target)
                        k++;
                    else if(sum > target)
                        l--;
                    else{//sum == target
                        List<Integer> item = new ArrayList<Integer>();
                        item.add(num[i]);
                        item.add(num[j]);
                        item.add(num[k]);
                        item.add(num[l]);
                        if(!hashSet.contains(item)){
                            hashSet.add(item);
                            res.add(item);
                        }
                        k++;
                        l--;
                    }
                }
            }
            return res;
    }
}