4Sum.java

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 ArrayList<ArrayList<Integer>> fourSum(int[] num, int target) {
        ArrayList<ArrayList<Integer>> ret = new ArrayList<ArrayList<Integer>>();
        Arrays.sort(num);
        int length = num.length;
        for (int i = 0; i < length - 3; i++) {
            if (i > 0 && num[i] == num[i - 1]) continue;
            for (int j = i + 1; j < length - 2; j++) {
                if (j > i + 1 && num[j] == num[j - 1]) continue;
                int l = j + 1;
                int r = length - 1;
                while (l < r) {
                	int delta = num[i] + num[j] + num[l] + num[r] - target;
                    if (delta == 0) {
                    	if (l > j + 1 && num[l] == num[l - 1]) {
                    		l++;
                    		continue;
                    	}
                    	if (r < length - 1 && num[r] == num[r + 1]) {
                    		r--;
                    		continue;
                    	}
                        ArrayList<Integer> item = new ArrayList<Integer>();
                        item.add(num[i]);
                        item.add(num[j]);
                        item.add(num[l]);
                        item.add(num[r]);
                        ret.add(item);
                        l++;
                        r--;
                    } else if (delta < 0) {
                        l++;
                    } else {
                    	r--;
                    }
                }
            }
        }
        return ret;
    }
}