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)

The algorithm is similar to 3Sum.

public class Solution {
    public ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        ArrayList<ArrayList<Integer> > result = new ArrayList<ArrayList<Integer> >();
        int len = num.length;
        if(len >=4){
            // sort array first
            Arrays.sort(num); 
            for(int i = 0; i < len - 3; ++i){
                int j = i + 1;
                int sum = target - num[i];
                while(j < len - 2){
                    int mid = j + 1; 
                    int end = len - 1;
                    while(mid < end){
                        int tripleSum = num[j] + num[mid] + num[end];
                        if(tripleSum == sum){
                            ArrayList<Integer> oneSolution = new ArrayList<Integer>();
                            oneSolution.add(num[i]);
                            oneSolution.add(num[j]);
                            oneSolution.add(num[mid]);
                            oneSolution.add(num[end]);
                            result.add(oneSolution);                        
                        }
                        else if(tripleSum < sum){
                            ++mid;
                            continue;
                        }
                        else{
                            --end;
                            continue;
                        }                    
                        ++mid;
                        --end;
                        while(mid < end && num[mid - 1] == num[mid])
                            ++mid;
                        while(mid < end && num[end] == num[end + 1])
                            --end;
                    }
                    ++j;
                    while(j < len - 2 && num[j - 1] == num[j])
                        ++j;
                }
                while(i < len - 3 && num[i] == num[i + 1])
                    ++i;            
            }
        }
        return result;    
    }
}