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)

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

            }
            while (i<n-1&&num[i]==num[i+1]) ++i;
        }
        return ret;
    }
}