[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:

• 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)

Solution:

O(n^3)的Time Efficiency.

public class Solution {
    public List<List<Integer>> fourSum(int[] num, int target) {
        List<List<Integer>> result=new ArrayList<List<Integer>>();
        Arrays.sort(num);
        for(int i=0;i<num.length-3;++i){
            if(i>0&&num[i]==num[i-1])
                continue;
            for(int j=i+1;j<num.length-2;++j){
                if(j>i+1&&num[j]==num[j-1])
                    continue;
                int low=j+1;
                int high=num.length-1;
                while(low<high){
                    int sum=num[i]+num[j]+num[low]+num[high];
                    if(sum==target){
                        List<Integer> temp=new ArrayList<Integer>();
                        temp.add(num[i]);
                        temp.add(num[j]);
                        temp.add(num[low]);
                        temp.add(num[high]);
                        result.add(temp);
                        low++;
                        high--;
                        while(low<high&&num[low]==num[low-1])
                            low++;
                        while(low<high&&num[high]==num[high+1])
                            high--;
                    }
                    else if(sum<target){
                        low++;
                    }
                    else 
                        high--;                        
                }
            }
        }
        return result;
    }
}