[LeetCode] 3Sum

Problem:

Given an array S of n integers, are there elements a, b, c in S such that a + b + c = 0? Find all unique triplets in the array which gives the sum of zero.

Java Code:

public class Solution {
    public ArrayList<ArrayList<Integer>> threeSum(int[] num) {
        ArrayList<ArrayList<Integer>> result = new ArrayList<ArrayList<Integer>>();
        if (num == null || num.length == 0) {
            return result;
        }
        
        Arrays.sort(num);
        
        for (int i = 0; i < num.length; i++) {
            if (i != 0 && num[i] == num[i - 1]) {
                continue;
            }
            int start = i + 1;
            int end = num.length - 1;
            while (start < end) {
                if (num[i] + num[start] + num[end] == 0) {
                    ArrayList<Integer> temp = new ArrayList<Integer>();
                    temp.add(num[i]);
                    temp.add(num[start]);
                    temp.add(num[end]);
                    result.add(temp);
                    start++;
                    end--;
                    while (start < end && num[start] == num[start - 1]) {
                        start++;
                    }
                    while (start < end && num[end] == num[end + 1]) {
                        end--;
                    }
                } else if (num[i] + num[start] + num[end] > 0) {
                    end--;
                } else {
                    start++;
                }
            }
        }
        return result;
    }
}